Sum Subarray Minimums

0907-Sum-of-Subarray-Minimums.py

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+'''
+Given an array of integers A, find the sum of min(B), where B ranges over every (contiguous) subarray of A.
+
+Since the answer may be large, return the answer modulo 10^9 + 7.
+
+
+
+Example 1:
+
+Input: [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.
+
+
+
+Note:
+
+    1 <= A.length <= 30000
+    1 <= A[i] <= 30000
+'''
+# Time Complexity: O(n)
+# Space Complexity: O(n)
+class Solution:
+    def sumSubarrayMins(self, A: List[int]) -> int:
+        res = 0
+        s = []
+        A = [0] + A + [0]
+        for idx, a in enumerate(A):
+            while s and A[s[-1]] > a:
+                i = s.pop()
+                j = s[-1]
+                res += A[i] * (idx - i) * (i - j)
+            s.append(idx)
+        return res % (10 ** 9 + 7)