LeetCode907. Sum of Subarray Minimums——单调栈

一、题目

Given an array of integers arr, find the sum of min(b), where b ranges over every (contiguous) subarray of arr. Since the answer may be large, return the answer modulo 109 + 7.

Example 1:

Input: arr = [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.
Example 2:

Input: arr = [11,81,94,43,3]
Output: 444

Constraints:

1 <= arr.length <= 3 * 104
1 <= arr[i] <= 3 * 104

二、题解

class Solution {
public:
    int sumSubarrayMins(vector<int>& arr) {
        int mod = 1e9 + 7;
        long long res = 0;
        int n = arr.size();
        stack<int> st;
        for(int i = 0;i < n;i++){
            while(!st.empty() && arr[i] <= arr[st.top()]){
                int cur = st.top();
                st.pop();
                int left = st.empty() ? -1 : st.top();
                res = (res + (long long)(cur - left) * (i - cur) * arr[cur]) % mod;
            }
            st.push(i);
        }
        while(!st.empty()){
            int cur = st.top();
            st.pop();
            int left = st.empty() ? -1 : st.top();
            res = (res + (long long)(cur - left) * (n - cur) * arr[cur]) % mod;
        }
        return (int) res;
    }
};