Add solution #907

Author: JoshCrozier

Diff for: README.md

@@ -717,6 +717,7 @@
 904|[Fruit Into Baskets](./0904-fruit-into-baskets.js)|Medium|
 905|[Sort Array By Parity](./0905-sort-array-by-parity.js)|Easy|
 906|[Super Palindromes](./0906-super-palindromes.js)|Hard|
+907|[Sum of Subarray Minimums](./0907-sum-of-subarray-minimums.js)|Medium|
 909|[Snakes and Ladders](./0909-snakes-and-ladders.js)|Medium|
 912|[Sort an Array](./0912-sort-an-array.js)|Medium|
 914|[X of a Kind in a Deck of Cards](./0914-x-of-a-kind-in-a-deck-of-cards.js)|Medium|

Diff for: solutions/0907-sum-of-subarray-minimums.js

@@ -0,0 +1,43 @@
+/**
+ * 907. Sum of Subarray Minimums
+ * https://leetcode.com/problems/sum-of-subarray-minimums/
+ * Difficulty: Medium
+ *
+ * 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.
+ */
+
+/**
+ * @param {number[]} arr
+ * @return {number}
+ */
+var sumSubarrayMins = function(arr) {
+  const MOD = 1e9 + 7;
+  const left = new Array(arr.length);
+  const right = new Array(arr.length);
+  const stack = [];
+
+  for (let i = 0; i < arr.length; i++) {
+    while (stack.length && arr[stack[stack.length - 1]] > arr[i]) {
+      stack.pop();
+    }
+    left[i] = stack.length ? i - stack[stack.length - 1] : i + 1;
+    stack.push(i);
+  }
+
+  stack.length = 0;
+  for (let i = arr.length - 1; i >= 0; i--) {
+    while (stack.length && arr[stack[stack.length - 1]] >= arr[i]) {
+      stack.pop();
+    }
+    right[i] = stack.length ? stack[stack.length - 1] - i : arr.length - i;
+    stack.push(i);
+  }
+
+  let result = 0;
+  for (let i = 0; i < arr.length; i++) {
+    result = (result + arr[i] * left[i] * right[i]) % MOD;
+  }
+
+  return result;
+};