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Sum of Subarray Minimums
Difficulty: Medium
Category: DSA
Topics: Array, Dynamic Programming, Stack, Monotonic Stack
Asked at: Google, Dunzo, Microsoft, VMware
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.
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**Example 2:**
**Input:** arr = [11,81,94,43,3]
**Output:** 444
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**Constraints:**
- `1 <= arr.length <= 3 * 104`
- `1 <= arr[i] <= 3 * 104`
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