Given an array of n elements such that elements may repeat. We can delete any number of elements from array. The task is to find minimum number of elements to be deleted from array to make it equal.

Examples:

Input : arr[] = {4, 3, 4, 4, 2, 4} Output : 2 After deleting 2 and 3 from array, array becomes arr[] = {4, 4, 4, 4} Input : arr[] = {1, 2, 3, 4, 5} Output: 4 We can delete any four elements from array.

In this problem we need to minimize the delete operations. The approach is simple, we count frequency of each element in array, then find the frequency of most frequent element in **count array**. Let this frequence be max_freq. To get the minimum number of elements to be deleted from array calculate **n – max_freq** where n is number of elements in given array.

`// C++ program to find minimum number of deletes required ` `// to make all elements same. ` `#include<bits/stdc++.h> ` `using` `namespace` `std; ` ` ` `// Function to get minimum number of elements to be deleted ` `// from array to make array elements equal ` `int` `minDelete(` `int` `arr[],` `int` `n) ` `{ ` ` ` `// Create an hash map and store frequencies of all ` ` ` `// array elements in it using element as key and ` ` ` `// frequency as value ` ` ` `unordered_map<` `int` `, ` `int` `> freq; ` ` ` `for` `(` `int` `i=0; i<n; i++) ` ` ` `freq[arr[i]]++; ` ` ` ` ` `// Find maximum frequency among all frequencies. ` ` ` `int` `max_freq = INT_MIN; ` ` ` `for` `(` `auto` `itr = freq.begin(); itr != freq.end(); itr++) ` ` ` `max_freq = max(max_freq, itr->second); ` ` ` ` ` `// To minimize delete operations, we remove all ` ` ` `// elements but the most frequent element. ` ` ` `return` `n - max_freq; ` `} ` ` ` `// Driver program to run the case ` `int` `main() ` `{ ` ` ` `int` `arr[] = {4, 3, 4, 4, 2, 4}; ` ` ` `int` `n = ` `sizeof` `(arr)/` `sizeof` `(arr[0]); ` ` ` `cout << minDelete(arr, n); ` ` ` `return` `0; ` `} ` |

Output:

2

Time complexity : O(n)

__ Note :__ Here we can optimize the extra space to count frequency of each element to O(1) but for this we have to modify our original array. See this article.

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