From 6ca484e8f086f5afda6cebbea1fa7557260c5392 Mon Sep 17 00:00:00 2001
From: kamyu <kamyu104@users.noreply.github.com>
Date: Sun, 16 Sep 2018 19:30:19 +0800
Subject: [PATCH] Create sum-of-subarray-minimums.cpp

---
 C++/sum-of-subarray-minimums.cpp | 40 ++++++++++++++++++++++++++++++++
 1 file changed, 40 insertions(+)
 create mode 100644 C++/sum-of-subarray-minimums.cpp

diff --git a/C++/sum-of-subarray-minimums.cpp b/C++/sum-of-subarray-minimums.cpp
new file mode 100644
index 000000000..76e6b6b48
--- /dev/null
+++ b/C++/sum-of-subarray-minimums.cpp
@@ -0,0 +1,40 @@
+// Time:  O(n)
+// Space: O(n)
+
+// Ascending stack solution
+class Solution {
+public:
+    int sumSubarrayMins(vector<int>& A) {
+        static const int M = 1e9 + 7;
+
+        vector<int> left(A.size());
+        stack<pair<int, int>> s1;
+        for (int i = 0; i < A.size(); ++i) {
+            int count = 1;
+            while (!s1.empty() && s1.top().first > A[i]) {
+                count += s1.top().second;
+                s1.pop();
+            }
+            s1.emplace(A[i], count);
+            left[i] = count;
+        }
+
+        vector<int> right(A.size());
+        stack<pair<int, int>> s2;
+        for (int i = A.size() - 1; i >= 0; --i) {
+            int count = 1;
+            while (!s2.empty() && s2.top().first >= A[i]) {
+                count += s2.top().second;
+                s2.pop();
+            }
+            s2.emplace(A[i], count);
+            right[i] = count;
+        }
+
+        int result = 0;
+        for (int i = 0; i < A.size(); ++i) {
+            result = (result + A[i] * left[i] * right[i]) % M;
+        }
+        return result;
+    }
+};