907

Stack/907.md

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+## [907] Sum of Subarray Minimums
+
+#### Description
+
+[link](https://leetcode.com/problems/sum-of-subarray-minimums/)
+
+---
+
+#### Solution
+
+- See Code
+
+---
+
+#### Code
+
+> O(n)
+
+```python
+class Solution:
+    def sumSubarrayMins(self, A: List[int]) -> int:
+        # The times a number n occurs in the minimums is |left_bounday-indexof(n)| * |right_bounday-indexof(n)| 
+        # The total sum is sum([n * |left_bounday - indexof(n)| * |right_bounday - indexof(n)| for n in array]
+        res = 0
+        stack = []  #  non-decreasing 
+        A = [float('-inf')] + A + [float('-inf')]
+        for i, n in enumerate(A):
+            # stack[-1], i is the left and right bounday
+            while stack and A[stack[-1]] > n:
+                cur = stack.pop()
+                res += A[cur] * (i - cur) * (cur - stack[-1]) 
+            stack.append(i)
+        return res % (10**9 + 7)
+```