LeetCode [907] Sum of Subarray Minimums

Given an array of integers A, find the sum of min(B), where B ranges over every (contiguous) subarray of A.

Since the answer may be large, return the answer modulo 10^9 + 7.

Example 1:

Input: [3,1,2,4]
Output: 17
Explanation: Subarrays are [3], [1], [2], [4], [3,1], [1,2], [2,4], [3,1,2], [1,2,4], [3,1,2,4]. 
Minimums are 3, 1, 2, 4, 1, 1, 2, 1, 1, 1.  Sum is 17.

Note:

1. 1 <= A.length <= 30000
2. 1 <= A[i] <= 30000

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class Solution { const int mod = 1e9+7; public: int sumSubarrayMins(vector<int> &A) { stack<pair<int, int>> stk; //val, count(#subset(ending at val) with min(subset)==val) int sum = 0; int partialSum = 0;//sum of min(B) ending at a for (auto a : A)//consider all subsets ending at 'a' { int count = 1;//min({a})==a, so count = 1 while (!stk.empty() && a < stk.top().first) { pair<int, int> vc = stk.top(); stk.pop(); partialSum -= vc.first * vc.second; count += vc.second; } stk.push(pair<int, int>(a, count)); partialSum += a * count; sum += partialSum; sum %= mod; } return sum; } };