Check if two arrays are equal or not

Difficulty: Easy

Asked in: Amazon, Goldman Sachs

Given two given arrays of equal length, Check if given arrays are equal or not.

Problem Note:

  • Two arrays are said to be equal if both of them contain the same set of elements, the permutation of the elements may be different though.
  • If there are repetitions, then counts of repeated elements must also be the same for two arrays to be equal.

Example 1

Input: A[] = [3, 8, 9, 1] , B[] = [9, 3, 1, 8]  
Output: 1

Example 2

Input: A[] = [1, 2, 5, 2, 1] , B[] = [2, 1, 2, 1, 5] 
Output: 1

Example 3

Input: A[] = [1, 2, 2, 7, 1] , B[] = [2, 1, 2, 7, 5] 
Output: 0

Example 4

Input: A[] = [0, 5, 0] , B[] = [5, 0, 5] 
Output: 0


We will be discussing two different solutions for this problem

  1. Sorting the array: Sort both the arrays and in a single traversal compare their values at each index.
  2. Using hash: Create a hash table or hash map for both the arrays that will contain the values and their occurrences in the arrays and compare them.

You may now try to solve this problem from here . by applying the above two approach. If stuck, then you can find the solution below:

1. Sorting The Array

To check if two arrays are equal or not, we have to compare the exact occurrence of each of the elements in both of the arrays to be the same.

However, the problem is that the values of the arrays could be in any permutation irrespective of each other.

If we could order them in any way, then it would be easy for us to check their number of occurrences. So, we will sort both the arrays and linearly traverse on it while comparing each element of the arrays. If at any indices after sorting they mismatch then we could return false from there otherwise, we’ll return true .

Solution Steps

  • Compare the lengths of arr1 and arr2 . If they are equal, then we will continue otherwise return false .
  • Sort arr1 and arr2 either in ascending or descending order.
  • For each index i of the array, compare arr1[i] and arr2[i] to be equal.
  • If at any point the condition fails, then return False otherwise, at the end of the comparison, return True .

Pseudo Code

bool twoArrEqual(int arr1[], int arr2[]) 
    // If lengths of array are not equal means 
    // array are not equal 
    if (arr1.length != arr2.length) 
        return false
    // Sort both arrays 
    // Linearly compare elements 
    for (int i = 0 to i < arr1.length) 
        if (arr1[i] != arr2[i]) 
            return false
    // If all elements were same. 
    return true

Complexity Analysis

Time Complexity: O(n log n) using merge sort. Using another quadratic sorting algorithm will make the time complexity O(n²)

Space Complexity: O(1)

Critical Ideas to Think

  • Do you think that there could be another approach in which we will take each permutation of elements in the first array and compare it with the second array? If that could be a valid solution, then what would be its complexity?
  • Which sorting algorithm should be used here to sort the arrays?(comment down below)
  • Can you optimize the time complexity?

2. Using Hash

If we will think again, we could further optimize the time complexity of the previous approach, if you will notice that only the count of occurrence of each element is relevant to us to check the equality of the two arrays.

So, If there exists a value in arr1 then it has to exist in arr2 with the same number of occurrences in each of them.

This reminds me of a data structure called HashMap also known as HashTable or Dictionary . We can use a HashMap to store the elements and their occurrence. Now we can simply iterate over the second array and reduce the count by one for each element.

Solution steps

  • Check the size of both the arrays
  • Create a HashMap for arr1 and store the count for each element.
  • Iterate over arr2 and check for each element in arr2 there should be a key in HashMap and its value corresponding to the key must be greater than 0.
  • If such key exists, then decrease the count by one for that key in that particular iteration
  • If the value of the key is not greater than 0, return False
  • At the end of the code return True .

Pseudo Code

bool twoArrEqual(int arr1[], int arr2[]) { 
    // If lengths of arrays are not equal 
    n = arr1.length
    m = arr2.length
    if (n != m) 
        return false; 
    // Store arr1[] elements and their counts in hash map
    HashMap map
    for (int i = 0 to i < n) 
        if(arr1[i] in map.keys())
            map[arr1[i]] = map[arr1[i]] + 1
            map[arr1[i]] = 1
    // if all elements of arr2[] are present same number 
    // of times or not. 
    for (int i = 0 to i < n ) { 
        if (arr2[i] not in map.keys()) 
            return false
        // If an element of arr2[] appears more 
        // times than it appears in arr1[] 
        if (map[arr2[i]] == 0)
            return false
        map[arr2[i]] = map[arr2[i]] - 1
    return true

Complexity Analysis

Time Complexity: O(n). You can use an unordered map. Using Ordered map might land time complexity to O(n log n) for large values of n.

Space Complexity: O(n)

Critical Ideas to Think

  • If an element of arr1 appears more than that in arr2 , then do you think that this algorithm will work? If yes, How?
  • Can you think of the case when map[arr2[i]] become 0 for any i ?
  • Instead of sorting and using auxiliary space, can you think of any other approach in O(n²) time complexity?

Comparison of Different Approaches

Suggested Problems to Solve

  • Check whether it is possible to make both arrays equal by modifying a single element
  • Find sub-arrays from given two arrays such that they have equal sum
  • Count sub-arrays which have elements less than or equal to X
  • Count pairs from two sorted arrays whose sum is equal to a given value x

If you have any more approaches or you find an error/bug in the above solutions, please comment down below.

Happy Coding! Enjoy Algorithms!