# Sum of all even occurring element in an array

Given an array of integers containing duplicate elements. The task is to find the sum of all even occurring elements in the given array. That is the sum of all such elements whose frequency is even in the array.

Examples:

```Input : arr[] = {1, 1, 2, 2, 3, 3, 3}
Output : 6
The even occurring element is 1 and 2, and it's number
of occurrence is 2. Therefore sum of all 1's in the
array = 1+1+2+2 = 6.

Input : arr[] = {10, 20, 30, 40, 40}
Output : 80
Element with even frequency are 40.
Sum = 40.
```

## Recommended: Please try your approach on {IDE} first, before moving on to the solution.

Approach:

• Traverse the array and use a unordered_map in C++ to store the frequency of elements of the array such that the key of map is the array element and value is its frequency in the array.
• Then, traverse the map to find the frequency of elements and check if it is even, if it is even, then add this element to sum.

Below is the implementation of the above approach:

 `// CPP program to find the sum of all even ` `// occurring elements in an array ` ` `  `#include ` `using` `namespace` `std; ` ` `  `// Function to find the sum of all even ` `// occurring elements in an array ` `int` `findSum(``int` `arr[], ``int` `N) ` `{ ` `    ``// Map to store frequency of elements ` `    ``// of the array ` `    ``unordered_map<``int``, ``int``> mp; ` ` `  `    ``for` `(``int` `i = 0; i < N; i++) { ` `        ``mp[arr[i]]++; ` `    ``} ` ` `  `    ``// variable to stroe sum of all ` `    ``// even occurring elements ` `    ``int` `sum = 0; ` ` `  `    ``// loop to iteratre through map ` `    ``for` `(``auto` `itr = mp.begin(); itr != mp.end(); itr++) { ` ` `  `        ``// check if frequency is even ` `        ``if` `(itr->second % 2 == 0)  ` `            ``sum += (itr->first) * (itr->second);  ` `    ``} ` ` `  `    ``return` `sum; ` `} ` ` `  `// Driver Code ` `int` `main() ` `{ ` `    ``int` `arr[] = { 10, 20, 20, 40, 40 }; ` ` `  `    ``int` `N = ``sizeof``(arr) / ``sizeof``(arr); ` ` `  `    ``cout << findSum(arr, N); ` ` `  `    ``return` `0; ` `} `

Output:

```120
```

Time Complexity: O(N), where N is the number of elements in the array.

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