I recently published an article Overview of Data Structures in Java Programming Language and ES6 JavaScript Data Structures you should Know, whose aim was to document the data structures available in Java and JavaScript programming languages, respectively, and the type of software engineering problems, where such data structures are useful.

While data structures are strongly related to algorithms, the latter presents interesting facts, that software engineers need to know to be good in problem-solving.

To build a large-scale computer system (such as those run by Google, Microsoft, Facebook, Netflix, Uber, Amazon, etc.), the concept of algorithms is crucial. So what is an algorithm?

An algorithm is a finite set of instructions that, when followed, accomplishes a particular task.

This definition is good, but it does touch on all attributes of a good algorithm. An algorithm is expected to meet the following characteristics;-

i) Input – An algorithm is supplied with either zero or more quantities.

ii) Output –  At least one quantity must be produced from algorithm steps.

iii) Finiteness – An algorithm must terminate after a finite number of steps.

iv) Non-ambiguous – Also called definiteness, the steps in an algorithm must be clear or mean only one thing.

v) Correctness –  The output from an algorithm must be correct or should be the intended output.

Apart from the characteristics, we have mentioned, all the algorithm instructions must be simple enough to be done by a person using a pen and paper.

Objectives of this walk-through

  • To highlight algorithm design strategies (categories of algorithms) by solving sample software engineering problems.
  • To highlight some tips you need when solving software engineering problems.

Who is this walk-through for?

  • Computer Science students.
  • Professional software engineers.
  • Software engineering job seekers.
  • Computer Science teachers/lecturers/professors


  • A basic understanding of data structures and algorithms.
  • Basic knowledge of Java programming language.

Now let us dive deeper and look at the things you must consider while trying to improve your software engineering skills. For each item we look at, we will solve a sample problem to illustrate our explanation.

1. Mastering data structures is a must.

You always interact with data when solving problems. Data structures provide a way to store complex and big datasets, enabling one to design efficient algorithms. Most programming languages have inbuilt data structures, and you should master data structures in the programming language that you love.

For Java and JavaScript engineers, you can check out these 2 (two) articles, which I published myself, Overview of Data Structures in Java Programming Language, and ES6 JavaScript Data Structures you should Know. It gives an extensive discussion of data structures you need to know.

Problem 1:

Given an unsorted array, return true if there are duplicates (has numbers appearing more than once), return false otherwise.


Input: {1, 2, 3, 1}

Output: true

Explanation: 1 appears twice.

Input: {1, 2, 3, 4}

Output: false

Explanation: Each of the elements in the array appear once, so there are no duplicates.

If you know nothing about data structures, you may solve the problem, but your solution will not be the most efficient.

The algorithm:

You will take each of the numbers in the array and check if it appears again. i.e. for  {1, 2, 3, 4}, compare 1 and 2,  1 and 3, 1 and 4; compare 2 and 3, 2 and 4; compare 3 and 4

We will use 2 for loops as shown below and the resultant algorithm has constant space complexity, O(n) and quadratic running time complexity, O(n^2)

public boolean hasDuplicates(int a []) {
      int n = a.length;	
      if (n <=1) {
         return false;
      for (int i = 0; i < a.length; i++) {
         for (int j = i+1; j < a.length; j++) {
             if (a[i] == a[j]) {
                return true;
      return false;

Can we solve this problem in linear time, O(n)?

Well, yes. We need to introduce a data structure called a set/HashSet. This data structure has 2 advantages;-

  • Does not accepts duplicates, so it will always contain unique values.
  • It affords constant look up time, so we will not waste any time when we want to check if a given value exists.

The solution below has linear run time, O(n) and linear space, O(n).

public boolean hasDuplicates(int a []) {
     int n = a.length;
     if (n <=1) {
        return false;
     Set<Integer> set = new HashSet<Integer>();
     for (int i = 0; i < a.length; i++) {
         if (set.contains(a[i])) {
            return true;
         }else {
     //If we successfully added all the elements into the set, then
     there are no duplicates
     return false;

There is a simple Leetcode problem called Two sum, which you can attempt and see if you can identify the appropriate data structure to use to solve the problem in linear time.

2. You must understand data types properly.

We are aware that data structures are mostly built from simple or primitives data types such as boolean, byte, char, short, int, long, float, and double.

The understanding we are talking about here is the structure of the data type, how it is stored in memory, how much space it occupies, value ranges (especially for numeric data types), etc.

For instance for a Java integer, int, the following needs to be known at the minimum;-

  • We use 32 bits to represent the number.
  • The number can be represented in bits.
  • For a signed 32-bit integer, it has a maximum value of 231-1 and a minimum of -231. An unsigned Java integer has a minimum of 0 and a maximum of 232-1. See more info here.
  • Java programming language gives you a way to access a signed integer maximum and minimum values as Integer.MAX_VALUE, and Integer.MIN_VALUE respectively.
  • You need to understand the bit-wise operations on integers.
  • If you have values ranges outside the provides range, you can use long data types, which uses 64 bits.

Problem 2:

You are given an integer array, nums of size n, and you are required to compute the sum of all the integers in the array and then return the sum.

Additional information:

  • -231<=nums[i]<= 231-1
  • 0<=n<= 1000

The algorithm

Computing the sum of integers in an array is a trivial task, so you must wonder why we are being asked this question. Look at the additional information;

  • The array can have big numbers, in fact, it has an integer, which is the biggest number an integer can accommodate, 231-1. This means that if the array has just two numbers, the resultant sum cannot be accommodated in an integer. This is possible because we are told that the array can be as big as 1000.

The trick here is to understand the additional information, and in our case, we will have to change our return type. If we write a function called solution, then it will be something like this;-

public long solution (int nums[]) {
     long sum = 0;
     for (int i = 0; i < nums.length; i++) {
        sum = sum + nums[i];
     return sum;

Notice that we return long instead of int – unsigned long.

The following solution looks okay but it will output a wrong result for some inputs. You can test it with this array {2,4,78,5,2147483647}

public int solution (int nums[]) {
    int sum = 0;
    for (int i = 0; i < nums.length; i++) {
        sum = sum + nums[i];
    return sum;

The correct sum is 2147483736 but it will output -2147483560!

3. Different programming languages may implement similar data structures differently.

I once again refer you to read the two articles Overview of Data Structures in Java Programming Language, and ES6 JavaScript Data Structures you should Know. You will notice that similar data structures will behave differently in different programming languages.

For instance, a Java array needs to be declared with size, whereas a JavaScript array expands to accommodate more items. Thus a JavaScript array can be used as an ordinary list, a queue, and at the same time, a stack.

In addition, a Java HashMap does guarantee that the order of items put in the hashmap will remain constant over time, but a JavaScript Map remembers the original insertion order of the keys.

Problem 3:

You are given an integer array, numsof size n, and you are required to store the values and their frequencies in some appropriate data structure. The order in which the values appeared in the array must be preserved.


input: {3,4,3,3,5,8,2,1,5,2,2,4,3}

output: 3,4; 4,2; 5,2; 8,1; 2,2; 1,1

Explanation: 3 appears thrice, 4 appears twice, 5 appears twice, 8 appears once, 2 appears twice and 1 appears once.

The algorithm

The idea is to think of a data structure that can store the information as key, value pair (the number is the key and frequency is the value), but at the same time maintain the insertion order.

With JavaScript, we can use a Map/HashMap. However, because Java HashMap does not maintain insertion order, we will need to use Java LinkedHashMap. You can see that Map/HashMap worked in JavaScript but not in Java.

Java solution:

//Java solution
public LinkedHashMap<Integer, Integer> solution (int nums []){
     LinkedHashMap<Integer, Integer> orderedMap = 
     new LinkedHashMap<Integer, Integer>();
     for (int i = 0; i < nums.length; i++) {
         if(orderedMap.containsKey(nums[i])) {
             orderedMap.put(nums[i], orderedMap.get(nums[i]) + 1);
         }else {
     return orderedMap;

JavaScript solution:

solution (nums){
    let orderedMap = new Map();
    for (let i = 0; i < nums.length; i++) {
          orderedMap.set(nums[i], orderedMap.get( nums[i]) + 1);
          orderedMap.set(nums[i], 1);
    return orderedMap;

4. Think of a brute force solution first, then optimize your algorithm

More often, software engineering problems will come with constraints, which guides the engineer in providing an efficient solution. However, it is advisable to provide a brute force (a naive approach) solution to a problem before optimizing it.

This is advantageous because it makes you understand the problem. Better still, if you were in an interview situation, providing a brute force solution is better than providing no solution at all.

Please do not rush to optimize because it may take longer, or you may even fail to crack the efficient solution.

Problem 4:

Compute the sum of all the integers between two numbers (the two numbers are included in the sum). It would help if you solved this in constant time, O(n). It is guaranteed that 10<=sum<=231-1


input: 2 and 10

output: 54

The algorithm

For sure, it might be challenging to figure out how to solve this problem in constant time, but as we have agreed, lets first provide a brute force solution.

We write a loop, which sums all integers between start, say 2(two) and end, say 10.

Something like this:

public int solution(int start, int end){
     int sum = 0;
     for(int i=start; i<=end; i++) {
        sum = sum + i;
     return sum;

This is a correct solution but its running time complexity is linear, O(n).

Next, let’s figure out how we can optimize this:

The sum of integers between 2 and 10 is done by adding all the numbers, including 2 and 10 i.e. 2+3+4+5+6+7+8+9+10.

If you are keen, what you see is the sum of an arithmetic progression, AP and we have a formula for it, s = n/2(a+l), where, n is the number of terms, a is the first term and l is the last term.

Thus, the solution with constant running time, O(1) is shown below;-

//Time complexity- O(1),
space complexity - O(1)public int solution(int start, int end){
     //Number of terms, n
     int n = (end - start) + 1;
     return n * (start+end)/2;

To add unto this explanation, there is a post on finding a pair with a given sum in an array, which gives a step by step explanation on improving a naive approach to an efficient approach.

5. Characters are equally important, know their ASCII values

ASCII stands for American Standard Code for Information Interchange. Computers can only understand numbers, so an ASCII code is the numerical representation of a character such as 'a' or 'A' or special character such as '@'.

Understanding the ASCII  codes can be helpful in solving certain problems. For instance, how do you know if an input is a lower case letter, upper case letter or a number and not a special character?

Problem 5:

Given a character input, determine if it is a number, upper case character, lower case character or a special character/other character.

The algorithm:

The approach we use is to check the ASCII value of the input. Thus, we need to know the following:

  • Upper case letter alphabets (A-Z) lie in the range 65-91 of the ASCII values
  • Lower case letter alphabets (a-z) lie in the range 97-122 of the ASCII values
  • Number 0 to 9 lie in the range 48-57.
  • Any other ASCII value is for a non-alphabetic character/non-numeric and it includes special characters.

We can thus write a function which returns a character description as shown below

public String checkCharacter (char input) {
     if (input >= 'A' && input <= 'Z') {
         return new String("It is upper case");
     }else if (input >= 'a' && input <= 'z') {
         return new String("It is lower case");
     }else if(input >='0' && input <='9') {
         return new String("It is a number");
     }else {
         return new String("Other character");

Another problem which you can solve using this technique is, Given a string, check if it is a Pangram or not. You must solve this problem in linear time, O(n)and constant space, O(1). You can find a comprehensive discussion and solution here.

6. You must love Math, it is at the heart of problem-solving

The art of designing and optimizing algorithms is aided by math.

If you go back to problem 4, you will notice that without understanding a number series, it would have been nearly impossible to optimize our solution.

Not all algorithms you write will require strong math principals but understanding math topics such as number theory, recurrence relations, probability, calculus,  set theory, proof techniques etc… would be advantageous. In the next sections, you will see how we solve recurrence relations using the backward substitution method to compute the running time complexities for recursive algorithms.

Problem 6:

Given a positive integer n, determine the smallest positive number/integer m, which has the same number of digits as n.


n = 1567, m = 1000; both m and n have the same number of digits and m is the smallest in that group.

The algorithm:

The problem we have is to determine the number of digits on n and use it to compute the smallest value with the same number of digits – we  power 10 to the number of digits-1.

We can use a string-based approach i.e. convert the integer into a string, then use the length function to compute the number of digits. The solution would be something like this;-

public int solution (int n){
      if(n == 0) {
         return 0;
      int numberOfDigits = String.valueOf(n).length();
      return (int)Math.pow(10,numberOfDigits-1);

If we have a few values, this solution is okay but if the number of values grows, then it will be totally inefficient because the statement int numberOfDigits = String.valueOf(n).length(); involves memory allocation for a String, for each evaluation. In the case of Java language, JVM must first parse our number and copy its digits into a separate String and perform a number of different operations as well (like keeping temporary copies, handle Unicode conversions etc).

We could use the logarithm approach. If a is equal to b to the power c, i.e. bc = a, we also say that c is the logarithm of a to the base b (meaning c is the power to which we have to raise b in order to get a), and we write logb a = c.

Thus the number of digits can be obtained as follows:

int numberOfDigits = (int) (Math.log10(n) + 1);

Using this approach, the solution to our problem would be as shown below:-

public int solution (int n){
    if(n == 0) {
       return 0;
    int numberOfDigits = (int)Math.log10(n) + 1;
    return (int)Math.pow(10,numberOfDigits-1);

Now you see why you have to love Math!

7. Take additional information or constraints seriously!

When you are given a problem to solve, more often it will include additional information or constraints. A good example is problem 2, which we solved earlier (recall how the additional information changed the way we designed the solution).

Additional information can describe the nature of data you expect or it gives you a clue on how how to solve the problem.

Problem 7:

Let us see a modified version of a problem, which appears as number 1 in Leetcode:

Given an array of integers, nums of size n and an integer target, return the indices of the two numbers such that they add up to target.

You may assume that each input would have exactly one solution, and you may not use the same element twice.

You can return the answer in any order. Can you solve this problem in linear time, O(n) and constant space, O(1).


Input: nums = [2,7,11,15], target = 9

Output: [0,1]

Explanation: Because nums[0] + nums[1] == 9, so we return [0, 1]

Constraints/Additional information

  • 1 <= nums.length <= 105
  • -109 <= nums[i] <= 109
  • -109 <= target <= 109
  • Only one valid answer exists.
  • Array nums is sorted

The algorithm:

The question is easy to understand. There is additional information and some information is given in the problem description. Most importantly, we are told to solve this problem in linear time, O(n) and constant space, O(1), we also know that the array is sorted and that a valid answer exists in the input array.

We can solve this problem in a number of ways:

  • Generate all possible pairs and pick the one whose values add up to the target. Unfortunately, this solution ignores that fact that the array is sorted and its running time complexity is O(n2)
  • Traverse the array and store target-nums[i] as key and ias the value in a hash map. At any point of the traversal check if nums[i] exists in the hash map and if it does, you know you got the pair. Unfortunately, this solution ignores that fact that the array is sorted and its space complexity is O(n)
  • Because the array is sorted, we know the smaller number is on the left side of the bigger number (or are side by side if the two numbers are equal). We use two pointers, rPointer and lPointer initialized at n-1 and 0 respectively, (we will see the 2-pointer approach in the next sections), which moves as follows: if nums[lPointer] + nums[rPointer] == target, we found the pair, if nums[lPointer] + nums[rPointer] > target, decrement the rPointer, otherwise, we increment the lPointer . Also lPointer is always less than rPointer.

The preferred solution will look like as shown below.

//Linear time and constant space.
public int[] solution (int nums [], int target, int n) {
    int lPointer = 0;
    int rPointer = n-1;
    int [] output = new int[2];
    while (lPointer < rPointer) {
        if (nums[lPointer] + nums[rPointer] == target) {
           output[0] = lPointer;
           output[1] = rPointer;
           return output;
        }else if (nums[lPointer] + nums[rPointer] > target) {
        }else {
    return output;

8. You must show the running time complexity and space complexity of your solutions

Running time complexity and space complexity is more often a daunting topic to starters in data structures and algorithms but it becomes clear when you understand its role and with enough practice.

In fact, running time and space complexity for certain algorithms should be at your fingertips i.e. it should be straightforward to answer even if you woke up from a deep sleep.

A simple teaser

Can you write down the complexities of the following operations (Just stick to worst-case and use Big Oh notation). Each item should not take you more than 30 seconds

  • Running time and space complexity of bubble sort?
  • Running time and space complexity of binary search?
  • Running time and space complexity determining the largest number in a sorted integer array
  • Running time and space complexity of finding a value on a map
  • Running time and space complexity of merge sort and quick sort
  • Running time and space complexity of determining if a sorted array has duplicates.
  • Running time and space complexity if the unsorted array has duplicates.
  • Running time complexity of node deletion in a doubly-linked list.

Alternatively, can you give a known example of an algorithm with the following running time complexities (can you do it in less than 30 minutes for each item)?

  • O(1)Constant
  • O(log n)Logarithmic
  • O(n)Linear
  • O(n log n)Linearithmic
  • O(n2)Quadratic
  • O(2n)Exponential

Did you manage? If you did not, then you will need to go back and understand this topic better. If you did, you still need to know how to arrive at the answer.

Let us try an algorithm example, which gives us O(n2) and another with O(log n) running time complexities.

Refer to problem 1’s first solution, which had quadratic running time before we optimized it. It is shown below:

public boolean hasDuplicates(int a []) {
    int n = a.length;
    if (n <=1) {
       return false;
    for (int i = 0; i < a.length; i++) {
       for (int j = i+1; j < a.length; j++) {
          if (a[i] == a[j]) {
              return true;
    return false;

Using the example {1, 2, 3, 4, 5} as input, it has no duplicates, so we will experience a worst case for this algorithm.

Size of the array (input), n is 5

We will perform the following comparisons,

Pass 1:

Compare 1 and 2,  1 and 3, 1 and 4, 1 and 5 => 4 comparisons (expressed as n-1 comparisons )

Pass 2:

compare 2 and 3, 2 and 4, 2 and 5 => 3 comparisons (expressed as n-2 comparisons)

Pass 3:

Compare 3 and 4, 3 and 5 = > 2 comparisons (expressed as n-3 comparisons)

Pass 4:

Compare 4 and 5 => 1 comparison (expressed as n-4 comparisons)

You notice that we performed 4 passes (done by the i loop in the algorithm), when n is 5 (you can try more examples and you realize you will perform n-1 passes).

In each pass, we perform comparisons (see the j loop in the algorithm) and you can see that the number of comparisons decreases as we increase i. Let us summarize our stats in a table.

For n=5 (You can try this with different values of n and you will notice a similar pattern)

Table a: What is the relationship between the number of passes and input n for n = 5?

Pass Comparisons Express number of comparisons in terms of n
1 4 n-1
2 3 n-2
3 2 n-3
4 1 n-4
…… ……. …….

You are aware that we basically count the number of operations in an algorithm, T(n) and then determine the relationship between these operations and the input i.e

T(n) = 4 + 3 + 2 + 1 for n = 5 —–— (i) or

T(n) = 5 + 4 + 3 + 2 + 1 for n = 6  —–— (ii) etc…

or generally

T(n) = n-1 + n-2 + n-3 + n-4 for n=5 —–— (iii) or

T(n) = n-1 + n-2 + ......+ 2 + 1 for any input n —–— (iv)

Have you noticed something? This is an arithmetic profession (AP) and we are trying to find the sum of terms in an AP, whose equation is as shown below.

Sn = T(n) = n (a + l)/2, where a is the first term and l(this is el)  is the last term in the series.

a = n-1 and l=1

Do the substitution and you will find that the final equation will be T(n) = n2 /2 (half n squared)

Extracting running time complexity: drop any constants and any lower terms, drop co-efficient of the highest term.

Running time complexity thus is O(n2).

Problem 8:

Let us see another example, which gives us logarithmic running time complexity.

You are given an integer n of the form 2k(k=1, 2, 3, 4, 5) and you are required to divide n continuously to stop when n becomes 1. Your algorithm should return the number of divisions. What is the running time and space complexity of your solution?

Additional information:

  • You are guaranteed that the input is of the form of 2k.


input: 32

output: 5


  • 32/2 = 16
  • 16/2 = 8
  • 8/2 = 4
  • 4/2=2
  • 2/2= 1 (we stop when n is reduced to 1; we have done 5 divisions)

We use a while loop as shown below:

public int solution (int n) {
    int counter = 0;
    while(n>1) {
        n = n/2;
    return counter;

Looking for the answer from the algorithm may be tricky but if you have been writing algorithms, you should already tell the answer. We use a table as we did before to see how different values of n behave.

Table a: What is the relationship between the input n and the number of primitive operations i.e number of divisions, counter

Input (n) No. of divisions, counter (T(n)) Express n in terms of T(n)
4 2 22
8 3 23
16 4 24
64 6 26
…… ……. …….

In a general equation, the relationship between T(n) and n is;-

n = 2T(n)

We need to make T(n) the subject of the formula.

Apply log (base 2) on both sides; log n = log 2T(n)

log n = T(n)log 2; log of 2 to base 2 is equal to 1

Thus T(n)= log n

Running time complexity: O(log n)

In the next steps, we will discuss how to use recurrence relations to compute running time complexity for recursive algorithms.

Apart from computing running time complexity, you should be able to give scenarios in your algorithm, which gives best case and worst case running times in your algorithm.

9. You must know the running time complexity and space complexity of inbuilt functions in a programming language you use

It is always advisable to use inbuilt functions in the programming languages you use. Unfortunately, it is at the same not advisable to use inbuilt functions for problems with certain constraints, if you do not understand how the inbuilt functions have been implemented internally.

Example of inbuilt functions in java, which you might have used frequently include;-

  • Array.fill() – fills an array with a specified value
  • Arrays.sort() – sorts an array
  • Math.max() – returns the max value from a given list.

Do you know the time complexities of the above functions?

Problem 9:

Given an unsorted integer array, nums of size n and an integer target, return two numbers from the array, which adds up to the target.

You may assume that each input would have exactly one solution, and you may not use the same element twice. You may also return the numbers in any order.

What is the running time complexity of your solution?


Input: nums = [2,11,7,15], target = 9

Output: [2,7] or [2,7] is acceptable.

Explanation: Because 2 + 7 = 9, so we return [7, 2] or [2, 7].

The algorithm:

Unlike in problem 7, we are not required to return indices so we can change the order/positions of the numbers in the original array.

We will sort the array using Arrays.sort(), then use 2-pointer approach to determine the two numbers, which add up to the provided target.

The solution is as shown below;-

public int[] solution (int nums [], int target, int n) {
    //sort the array
    int lPointer = 0; //left pointer
    int rPointer = n-1; //Right pointer
    int [] output = new int[2];
    while (lPointer < rPointer) {
       if (nums[lPointer] + nums[rPointer] == target) {
           output[0] = nums[lPointer];
           output[1] = nums[rPointer];
           return output;
       } else if (nums[lPointer] + nums[rPointer] > target) {
       }else {
    return output;

So what is the running time complexity of our solution?

Well, the answer is O(nlogn). But how?

Well, we used Arrays.sort(), which uses quick sort whose running time complexity of O(nlogn).

We then used 2 pointers to traverse the sorted array to track the numbers adding up to the target and that gives a linear time, O(n).

So the equation will look like this;

T(n) = O(nlogn) + O(n); the highest term of the two is O(nlogn) and hence it becomes the running time complexity of our solution.

You can see that if you did not know-how Arrays.sort() works, it wouldn’t be easy to determine the running time complexity of the solution.

Problem 10:

Given a string, phrase, check if it is a pangram or not. A pangram is a sentence containing every letter in the English Alphabet. You may assume that the input contains only letters of the alphabet, which are all lower case. What is the running time complexity of your algorithm?


Input: the quick brown fox jumps over the lazy dog

Output: false

Explanation: ‘l’, ‘z’, ‘y’ are missing


We will use the concept of ASCII values of characters, a concept we discussed earlier.

We know 'a'-'a' = 0, 'b'-'a'=1, 'c'-'a'=2...etc, so if we create an array of size 26, we can use such ASCII arithmetic to increment values in the array, not including empty spaces.

We initialize all array values with 0 using Arrays.fill() function.

The solution is as shown below;-

//See comments within the code.
public boolean solution (String phrase) {
      //Remove all white spaces
      String newPhrase = phrase.replaceAll("\\s", "").toLowerCase();

      //prefill the array with zeros
      int countingArray [] = new int[26];
      Arrays.fill(countingArray, 0);
      for (int i = 0; i < newPhrase.length(); i++) {
          //Increment a value at a given index whenever you see
          encounter that character
          countingArray[newPhrase.charAt(i)-'a'] =
          countingArray[newPhrase.charAt(i)-'a'] + 1;
      //Check to see if there is still a zero in the array
      //which means a given character was not encountered
      for(int number: countingArray) {
          if(number == 0) {
               //If you see a zero, then the string is not a Pangram
               return false;
      return true;

There are three main sections in the algorithm, but only one will be important in determining the running time complexity complexity.

(i) Filling the array of size 26 using Arrays.fill() – The array is of size 26 regardless of the size of phrase. Although we need to know that running time complexity of Arrays.fill() is O(n), where n is the size of the array, it does not have any effect since the size of the array is constant (for any phrase size).

(ii) Traversing the characters of the sentence – This process will depend on the size of the input, phrase thus its running time is linear, O(n). This is the only part, which determines the running time complexity.

(iii) Checking the array to see if there is still any zero – This part does not affect the running time for the same reason as in (i).

Thus, the running time complexity of the algorithm is O(n). Can you see that the space complexity of the algorithm is constant, O(1)?

10. Improve efficiency using early function termination.

We have used a function in all the algorithms we have written. It should be okay for me to assume that you understand the effect of the keyword  return in a function.

Simply put, a return statement ends the execution of a function. So what does it mean by “Improve efficiency using early function termination“. Let us use an example.

Problem 11:

Given an unsorted integer array nums of size n, and a target, write an algorithm to determine if the target is present in nums.

Additional information:

  • 10<=n<=2.1 * 109

The algorithm:

We see that the size of the array can be very big.

The algorithm is simple, traverse the array elements one by one to check if it has the target.

We will have two solutions as follows. Notice the position where the return statements appears in each of the solutions. The function returns the target or -1 if the target is not found.

public int findA (int nums [], int target, int n) {
      target = -1;
      for (int i=0; i<n; i++) {
          if (nums[i] == target) {
             return nums[i];
      return target;
public int findB (int nums [], int target, int n) {
     target = -1;
     for (int i=0; i<n; i++) {
         if (nums[i] == target) {
            target =  nums[i];
     return target;

If we had an array, whose target value is close to the beginning (best case scenario), findA would return the target and terminate quickly. Unfortunately, for findB, it does not matter the position of target, it will perform the same number of operations for both a target near the beginning and a target towards the end or no target at all (worst case).

Rule of thumb is, for an algorithm execution time (not necessarily running time complexity), if possible, the best case scenario should be better than worst case.

11. A 2-pointer or multiple pointer approach is useful

Two pointer or sometimes known as multiple pointer approach is a really easy and effective technique, which is typically used for searching pairs in a sorted array.

If you refer to problem 7, where we are supposed to return the indices of two numbers such that they add up to a target, we described 3 approaches and the third one, which uses the 2-pointer approach was the most efficient.

Another problem, where you can use a 2-pointer approach (which also looks like a sliding window technique) is in a Leetcode problem, Subarray Sum Equals K – You can try it out.

12. Know how to compute running time complexity for recursive algorithms

We will discuss recursion and dynamic programming techniques in another post. In this part though, I will show you how to develop recurrence relation from a recursive solution and how to solve the recurrence relation using backward substitution.

I just need to list the laws of recursion before we look at an example.

  • A recursive algorithm must have a base case.
  • A recursive algorithm must call itself, recursively.
  • A recursive algorithm must change its state and move toward the base case (recall where a function calls itself by passing a smaller version of the input)

Problem 12:

The algorithm for computing the factorial of a number, n!is shown below. Write down the recurrence relation, solve it and hence write down the running time complexity of the algorithm.

public int factorial(int n) {
     if (n == 1)
        return 1;
     return n * factorial(n - 1);


We can see that in the algorithm (also by definition), the base case in 1! = 1 or 0!=1,

Thus if T(n) represents running time complexity, then T(1) = 1 or T(0) = 1 (our base case)

If the running time complexity for input n is T(n), then that of n-1 is T(n-1)

We also see every time, we check the condition if (n == 1), which happens in constant time (say C) or multiplying n by result from n-1!, which also happens in constant time, say C.

Thus, the recurrence relation is:

T(n) = T(n-1) + C, we also know that T(1) = 1

Let’s solve it!

T(n) = T(n-1) + C .............................. (i)

If we know what T(n-1) is equivalent to, we can solve equation (i)

If we subtract 1 from n on both sides of the equation, we can see what T(n-1) is equivalent to.

T(n-1) = T(n-2) + C;  substitute this equation in i

T(n) = T(n-2) + 2C...............................(ii)

We still did not manage to eliminate T(n) on the right side of the equation.

Let’s find out what T(n-2) is equivalent to.

We subtract 2 from n on both sides of equation i

T(n-2) = T(n-3) + C;  substitute this equation in ii

T(n) = T(n-3) + 3C..................................(iii)

We would continue with backward substitution and you can guess that our next equation would be as shown below;-

T(n) = T(n-4) + 4C..................................(iv)

The effect of subtracting 1 continuously from n will result in n decreasing to 1.What we need to know is the number of operation, which will reduce n to 1.

Assuming we need k subtractions, then the general equation would be;-

T(n) = T(n-k) + kC (notice the trend in equations i - iv) and at this point (recursion base case), T(n-k) = T(1) = 1

Thus n-k = 1; k = n-1 or n=k+1

T(n) = 1 + C(n-1);

T(n) = 1 + Cn-C

Drop all constants ( 1 and -C) as well as coefficient of the highest term, C.

Running time complexity is O(n).

Problem 13:

Refer to problem 8, which we solved using iteration. Its recursive solution is as shown below;-

//counter is declared at class level
//base condition is n==1? is a condition to stop
public int solution (int n) {
      if (n == 1) {
         return counter;
      return solution(n/2);

Although the problem does not merit to be solved using recursion, we did it for illustration purposes.

How will the recurrence relation look like?

T(n) = T(n/2) + C...............................................(i)

What is T(n/2) equivalent to?

T(n/2) = T(n/4) + C (we divided n by 2 on both sides of equation i). Substitute this equation in i.

T(n) = T(n/4) + 2C can be written as T(n) = T(n/22) + 2C.........(ii)

What is T(n/4) equivalent to?

T(n/4) = T(n/8) + C, we divide n by 4 on both sides of equation i). Substitute this equation in ii.

T(n) = T(n/8) + 3C can be written as T(n) = T(n/23) + 3C.........(iii)

We can go on and on doing backward substitution but what is our base case?

We know that when n=2, we need 1 division, hence T(2) = 1 (or still if n = 1, T(1) = 1).

How many divisions do we need to reach the base case? Assuming that we need k divisions, then the general recurrence equation representation is;-

T(n) = T(n/2k) + kC (see the pattern in equation i - iii) .................(iv)

But, T(2) = 1 =  T(n/2k)

Thus, 2 = n/2k; 2k+1 = n; k = logn -1 substitute in equation iv

T(n) = 1 + Clogn - C

Running time complexity is O(logn). Similar to the iterative algorithm.

There is another example in the following video, which will cement your understanding of solving recurrence relations using backward substitution.

The objective of this walk-through was to review algorithm concepts and consequently solve problems, which uses the concepts.

But you can see this post is getting too long! We will cover the following concepts in another post:-

  • More on recursive algorithms
  • Dynamic programming algorithms
  • Moving sum approach
  • Identifying algorithm edge cases
  • Sliding window approach
  • Backtracking algorithm technique
  • Divide and conquer technique
  • The greedy approach
  • And more…..

I hope you enjoyed reading through this walk-through and it was helpful.

Do not hesitate to leave your questions in the comments section.

See you in the next post!