GeeksforGeeks App
Open App
Browser
Continue

# C++ Program for Least frequent element in an array

Given an array, find the least frequent element in it. If there are multiple elements that appear least number of times, print any one of them.
Examples :

```Input : arr[] = {1, 3, 2, 1, 2, 2, 3, 1}
Output : 3
3 appears minimum number of times in given
array.

Input : arr[] = {10, 20, 30}
Output : 10 or 20 or 30```

A simple solution is to run two loops. The outer loop picks all elements one by one. The inner loop finds frequency of the picked element and compares with the minimum so far. Time complexity of this solution is O(n2)
A better solution is to do sorting. We first sort the array, then linearly traverse the array.

## C++

 `// CPP program to find the least frequent element``// in an array.``#include ``using` `namespace` `std;`` ` `int` `leastFrequent(``int` `arr[], ``int` `n)``{``    ``// Sort the array``    ``sort(arr, arr + n);`` ` `    ``// find the min frequency using linear traversal``    ``int` `min_count = n+1, res = -1, curr_count = 1;``    ``for` `(``int` `i = 1; i < n; i++) {``        ``if` `(arr[i] == arr[i - 1])``            ``curr_count++;``        ``else` `{``            ``if` `(curr_count < min_count) {``                ``min_count = curr_count;``                ``res = arr[i - 1];``            ``}``            ``curr_count = 1;``        ``}``    ``}``  ` `    ``// If last element is least frequent``    ``if` `(curr_count < min_count)``    ``{``        ``min_count = curr_count;``        ``res = arr[n - 1];``    ``}`` ` `    ``return` `res;``}`` ` `// driver program``int` `main()``{``    ``int` `arr[] = {1, 3, 2, 1, 2, 2, 3, 1};``    ``int` `n = ``sizeof``(arr) / ``sizeof``(arr[0]);``    ``cout << leastFrequent(arr, n);``    ``return` `0;``}`

Output:

`3`

Time Complexity : O(n Log n)
Auxiliary Space : O(1)
An efficient solution is to use hashing. We create a hash table and store elements and their frequency counts as key value pairs. Finally we traverse the hash table and print the key with minimum value.

## C++

 `// CPP program to find the least frequent element``// in an array.``#include ``using` `namespace` `std;`` ` `int` `leastFrequent(``int` `arr[], ``int` `n)``{``    ``// Insert all elements in hash.``    ``unordered_map<``int``, ``int``> hash;``    ``for` `(``int` `i = 0; i < n; i++)``        ``hash[arr[i]]++;`` ` `    ``// find the min frequency``    ``int` `min_count = n+1, res = -1;``    ``for` `(``auto` `i : hash) {``        ``if` `(min_count >= i.second) {``            ``res = i.first;``            ``min_count = i.second;``        ``}``    ``}`` ` `    ``return` `res;``}`` ` `// driver program``int` `main()``{``    ``int` `arr[] = {1, 3, 2, 1, 2, 2, 3, 1};``    ``int` `n = ``sizeof``(arr) / ``sizeof``(arr[0]);``    ``cout << leastFrequent(arr, n);``    ``return` `0;``}`

Output:

`3`

Time Complexity : O(n)
Auxiliary Space : O(n)

Please refer complete article on Least frequent element in an array for more details!

My Personal Notes arrow_drop_up