907. Sum of Subarray Minimums

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`

907. Sum of Subarray Minimums
``````struct Solution;

impl Solution {
fn sum_subarray_mins(a: Vec<i32>) -> i32 {
let n = a.len();
let mut sum = 0;
let mut left: Vec<usize> = vec![0; n];
let mut right: Vec<usize> = vec![0; n];
let mut stack: Vec<usize> = vec![];
for i in 0..n {
left[i] = i + 1;
right[i] = n - i;
while let Some(j) = stack.pop() {
if a[j] < a[i] {
stack.push(j);
break;
} else {
right[j] = i - j;
}
}
if let Some(&j) = stack.last() {
left[i] = i - j;
}
stack.push(i);
}
for i in 0..n {
sum += (left[i] * right[i]) as i32 * a[i];
sum %= 1_000_000_007;
}
sum
}
}

#[test]
fn test() {
let a = vec![3, 1, 2, 4];
let res = 17;
assert_eq!(Solution::sum_subarray_mins(a), res);
}
``````