# Maximum possible sub-array sum after at most X swaps

Given an array arr[] of N integers and an integer X, the task is to find the maximum possible sub-array sum after applying at most X swaps.

Examples:

Input: arr[] = {5, -1, 2, 3, 4, -2, 5}, X = 2
Output: 19
Swap (arr, arr) and (arr, arr).
Now, the maximum sub-array sum will be (5 + 2 + 3 + 4 + 5) = 19

Input: arr[] = {-2, -3, -1, -10}, X = 10
Output: -1

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

Approach: For every possible sub-array, consider the elements which are not part of this sub-array as discarded. Now, while there are swaps left and the sum of the sub-array currently under consideration can be maximized i.e. the greatest element among the discarded elements can be swapped with the minimum element of the sub-array, keep updating the sum of the sub-array. When there are no swaps left or the sub-array sum cannot be further maximized, update the current maximum sub-array sum found so far which will be the required answer in the end.

Below is the implementation of the above approach:

 `// C++ implementation of the approach ` `#include ` `using` `namespace` `std; ` ` `  `// Function to return the maximum ` `// sub-array sum after at most x swaps ` `int` `SubarraySum(``int` `a[], ``int` `n, ``int` `x) ` `{ ` `    ``// To store the required answer ` `    ``int` `ans = -10000; ` ` `  `    ``// For all possible intervals ` `    ``for` `(``int` `i = 0; i < n; i++) { ` `        ``for` `(``int` `j = i; j < n; j++) { ` ` `  `            ``// Keep current ans as zero ` `            ``int` `curans = 0; ` ` `  `            ``// To store the integers which are ` `            ``// not part of the sub-array ` `            ``// currently under consideration ` `            ``priority_queue<``int``, vector<``int``> > pq; ` ` `  `            ``// To store elements which are ` `            ``// part of the sub-array ` `            ``// currently under consideration ` `            ``priority_queue<``int``, vector<``int``>, greater<``int``> > pq2; ` ` `  `            ``// Create two sets ` `            ``for` `(``int` `k = 0; k < n; k++) { ` `                ``if` `(k >= i && k <= j) { ` `                    ``curans += a[k]; ` `                    ``pq2.push(a[k]); ` `                ``} ` `                ``else` `                    ``pq.push(a[k]); ` `            ``} ` `            ``ans = max(ans, curans); ` ` `  `            ``// Swap at most X elements ` `            ``for` `(``int` `k = 1; k <= x; k++) { ` `                ``if` `(pq.empty() || pq2.empty() ` `                    ``|| pq2.top() >= pq.top()) ` `                    ``break``; ` ` `  `                ``// Remove the minimum of ` `                ``// the taken elements ` `                ``curans -= pq2.top(); ` `                ``pq2.pop(); ` ` `  `                ``// Add maximum of the ` `                ``// discarded elements ` `                ``curans += pq.top(); ` `                ``pq.pop(); ` ` `  `                ``// Update the answer ` `                ``ans = max(ans, curans); ` `            ``} ` `        ``} ` `    ``} ` ` `  `    ``// Return the maximized sub-array sum ` `    ``return` `ans; ` `} ` ` `  `// Driver code ` `int` `main() ` `{ ` `    ``int` `a[] = { 5, -1, 2, 3, 4, -2, 5 }, x = 2; ` `    ``int` `n = ``sizeof``(a) / ``sizeof``(a); ` ` `  `    ``cout << SubarraySum(a, n, x); ` ` `  `    ``return` `0; ` `} `

Output:
```19
```

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