Sum Of Subarray Minimums Problem
Description
LeetCode Problem 907.
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 10^9 + 7.
Example 1:
1
2
3
4
5
6
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:
1
2
Input: arr = [11,81,94,43,3]
Output: 444
Constraints:
- 1 <= arr.length <= 3 * 10^4
- 1 <= arr[i] <= 3 * 10^4
Sample C++ Code
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
class Solution {
public:
int sumSubarrayMins(vector<int>& arr) {
int n = arr.size(), mod = 1e9 + 7;
long sum = 0;
stack<pair<int, long>> sums;
for (int i = n - 1; i >= 0; i--) {
while (!sums.empty() && arr[i] <= arr[sums.top().first])
sums.pop();
if (sums.empty())
sums.push({i, (arr[i] * (n - i)) % mod});
else
sums.push({i, (sums.top().second + arr[i] * (sums.top().first - i)) % mod});
sum = (sum + sums.top().second) % mod;
}
return sum % mod;
}
};