Sum of Subarray Minimums
<p>Given an array of integers arr, find the sum of <code>min(b)</code>, where <code>b</code> ranges over every (contiguous) subarray of <code>arr</code>. Since the answer may be large, return the answer <strong>modulo</strong> <code>10<sup>9</sup> + 7</code>.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>

<pre>
<strong>Input:</strong> arr = [3,1,2,4]
<strong>Output:</strong> 17
<strong>Explanation:</strong> 
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.
</pre>

<p><strong class="example">Example 2:</strong></p>

<pre>
<strong>Input:</strong> arr = [11,81,94,43,3]
<strong>Output:</strong> 444
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>1 &lt;= arr.length &lt;= 3 * 10<sup>4</sup></code></li>
	<li><code>1 &lt;= arr[i] &lt;= 3 * 10<sup>4</sup></code></li>
</ul>
Stack; Array; Dynamic Programming; Monotonic Stack
use std::collections::VecDeque;

impl Solution {
    pub fn sum_subarray_mins(arr: Vec<i32>) -> i32 {
        let n = arr.len();
        let mut left = vec![-1; n];
        let mut right = vec![n as i32; n];
        let mut stk: VecDeque<usize> = VecDeque::new();

        for i in 0..n {
            while !stk.is_empty() && arr[*stk.back().unwrap()] >= arr[i] {
                stk.pop_back();
            }
            if let Some(&top) = stk.back() {
                left[i] = top as i32;
            }
            stk.push_back(i);
        }

        stk.clear();
        for i in (0..n).rev() {
            while !stk.is_empty() && arr[*stk.back().unwrap()] > arr[i] {
                stk.pop_back();
            }
            if let Some(&top) = stk.back() {
                right[i] = top as i32;
            }
            stk.push_back(i);
        }

        let MOD = 1_000_000_007;
        let mut ans: i64 = 0;
        for i in 0..n {
            ans += ((((right[i] - (i as i32)) * ((i as i32) - left[i])) as i64) * (arr[i] as i64))
                % MOD;
            ans %= MOD;
        }
        ans as i32
    }
}
