Create 1991-find-the-middle-index-in-array.js

Author: everthis

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

@@ -0,0 +1,21 @@
+/**
+ * @param {number[]} nums
+ * @return {number}
+ */
+const findMiddleIndex = function(nums) {
+  const n = nums.length, leftSum = Array(n + 1).fill(0)
+  const sum = nums.reduce((ac, e) => ac + e, 0) 
+  for(let i = 0; i < n; i++) {
+    leftSum[i+1] = leftSum[i] + nums[i]
+  }
+
+  let res
+  for(let i = 0; i < n; i++) {
+    if(leftSum[i] === sum - leftSum[i] - nums[i]) {
+      res = i
+      break
+    }
+  }
+
+  return res == null ? -1 : res
+};