Add solution #1991

Author: JoshCrozier

README.md

@@ -1,4 +1,4 @@
-# 1,672 LeetCode solutions in JavaScript
+# 1,673 LeetCode solutions in JavaScript
 
 [https://leetcodejavascript.com](https://leetcodejavascript.com)
 
@@ -1526,6 +1526,7 @@
 1985|[Find the Kth Largest Integer in the Array](./solutions/1985-find-the-kth-largest-integer-in-the-array.js)|Medium|
 1986|[Minimum Number of Work Sessions to Finish the Tasks](./solutions/1986-minimum-number-of-work-sessions-to-finish-the-tasks.js)|Medium|
 1987|[Number of Unique Good Subsequences](./solutions/1987-number-of-unique-good-subsequences.js)|Hard|
+1991|[Find the Middle Index in Array](./solutions/1991-find-the-middle-index-in-array.js)|Easy|
 1996|[The Number of Weak Characters in the Game](./solutions/1996-the-number-of-weak-characters-in-the-game.js)|Medium|
 2000|[Reverse Prefix of Word](./solutions/2000-reverse-prefix-of-word.js)|Easy|
 2011|[Final Value of Variable After Performing Operations](./solutions/2011-final-value-of-variable-after-performing-operations.js)|Easy|

solutions/1991-find-the-middle-index-in-array.js

@@ -0,0 +1,35 @@
+/**
+ * 1991. Find the Middle Index in Array
+ * https://leetcode.com/problems/find-the-middle-index-in-array/
+ * Difficulty: Easy
+ *
+ * Given a 0-indexed integer array nums, find the leftmost middleIndex (i.e., the smallest amongst
+ * all the possible ones).
+ *
+ * A middleIndex is an index where nums[0] + nums[1] + ... +
+ * nums[middleIndex-1] == nums[middleIndex+1] + nums[middleIndex+2] + ... + nums[nums.length-1].
+ *
+ * If middleIndex == 0, the left side sum is considered to be 0. Similarly, if
+ * middleIndex == nums.length - 1, the right side sum is considered to be 0.
+ *
+ * Return the leftmost middleIndex that satisfies the condition, or -1 if there is no such index.
+ */
+
+/**
+ * @param {number[]} nums
+ * @return {number}
+ */
+var findMiddleIndex = function(nums) {
+  let left = 0;
+  let right = nums.reduce((sum, n) => sum + n, 0);
+
+  for (let i = 0; i < nums.length; i++) {
+    left += nums[i];
+    right -= nums[i];
+    if (left - nums[i] === right) {
+      return i;
+    }
+  }
+
+  return -1;
+};