Find Most Frequent Element in an Array

“Find Most Frequent Element in an Array” is one of the favourite technical interview problem based on array data structutre. Here, we are given an array of ‘n’ elements and Our task is to find the most frequent element in an array. If there are multiple elements with same highest frequency, return the one element which appears first.

Example (find most frequent element in an array):

INPUT:
Arr[] = {1, 2, 3, 4, 2, 3, 4, 2}
OUTPUT:
2
EXPLANATION:
2 because it occurs most number of times

There are multiple methods to find the most frequent element in an array. Some of them are:

METHOD 1: Brute-Force Approach to find most frequent element in an array

The brute force solution of the given problem would be to run two nested loops and count the occurrence of each element of an array. While counting occurrences of an elements, store the value of highest frequent element in temporary variable. Finally, returns and print the temporary variable.

The time complexity of this solution is O(n²), where ‘n’ is the size of an array.

C++ Program to find Most Frequent Element in an Array is as follows:

/* C++ Program to find most frequent element in an array */
#include<bits/stdc++.h>
using namespace std;
void highestFrequent(int arr[],int n)
{
    int res = arr[0];
    int count = 1;
    for(int i = 0 ; i< n ; i++)
    {
        int temp_count = 0;
        for(int j = 0 ; j < n ; j++)
        {
            if(arr[i] == arr[j])
            {
                temp_count++;
            }
        }
        if(temp_count> count)
        {
            res = arr[i];
        }
    }    
    cout<<"The most frequent element of the array is "<<res<<"\n";
}
int main()
{
    int arr[8] = {1, 2, 3, 4, 2, 3, 4, 2};
    highestFrequent(arr,8);
}

OUTPUT:
The most frequent element of the array is 2

METHOD 2: Hashing Based Approach to find most frequent element in an array

The hashing based approach is much more time efficient approach than previous approach, but this approach uses extra auxiliary space.

Simply, store the frequency of all the elements of the array in hashmap.

Now, traverse the hashmap and return the element with highest frequency.

The time complexity of this solution is O(nlogn), where ‘n’ is the size of the array and ‘logn’ for inserting and searching in the hash map.

C++ Program to find most frequent Element in an Array is as follows:

/* C++ Program to find most frequent Element in an Array */
#include<bits/stdc++.h>
using namespace std;
void highestFrequent(int arr[],int n)
{
    /* Creating Hash Map*/
    map<int,int>mp;
    /* Inserting elements and count in Hashmap */
    for(int i = 0; i<n;i++)
    mp[arr[i]]++;
    map<int,int>:: iterator itr;
    int res = arr[0];
    int count = 1;
    /* Scanning Hashmap for most frequent element */
    for(itr = mp.begin(); itr!=mp.end() ; ++itr)
    {
        if(itr -> second > count)
        {
            count = itr->second;
            res = itr->first;
        }
    }
    cout<<"The most frequent element of the array is "<<res<<"\n";
}
int main()
{
    int arr[8] = {1, 2, 3, 4, 2, 3, 4, 2};
    highestFrequent(arr,8);
}

OUTPUT:
The most frequent element of the array is 2

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