1746. Maximum Subarray Sum After One Operation

You are given an integer array nums. You must perform exactly one operation where you can replace one element nums[i] with nums[i] * nums[i].

Return the maximum possible subarray sum after exactly one operation. The subarray must be non-empty.

class Solution {
public:
    int maxSumAfterOperation(vector<int>& A) {
        int n = A.size(), res = 0;
        vector<int> kadane(n,0);
        for(int i = 0, sum = 0; i < n; i++) {
            kadane[i] = sum = max(sum + A[i], 0);
        }
        for(int i = n - 1, sum = 0; i >= 0; i--) {
            res = max(res, A[i] * A[i] + (i ? kadane[i-1] : 0) + sum);
            sum = max(sum + A[i], 0);
        }
        return res;
    }
};