FindPivotIndex.js

// Source : https://leetcode.com/problems/find-pivot-index
// Author : Dean Shi
// Date   : 2018-01-29

/***************************************************************************************
 *
 * Given an array of integers nums, write a method that returns the "pivot" index of
 * this array.
 *
 * We define the pivot index as the index where the sum of the numbers to the left of
 * the index is equal to the sum of the numbers to the right of the index.
 *
 * If no such index exists, we should return -1. If there are multiple pivot indexes,
 * you should return the left-most pivot index.
 *
 * Example 1:
 * Input:
 * nums = [1, 7, 3, 6, 5, 6]
 * Output: 3
 * Explanation:
 * The sum of the numbers to the left of index 3 (nums[3] = 6) is equal to the sum of
 * numbers to the right of index 3.
 * Also, 3 is the first index where this occurs.
 *
 * Example 2:
 * Input:
 * nums = [1, 2, 3]
 * Output: -1
 * Explanation:
 * There is no index that satisfies the conditions in the problem statement.
 *
 * Note:
 * The length of nums will be in the range [0, 10000].
 * Each element nums[i] will be an integer in the range [-1000, 1000].
 *
 ***************************************************************************************/

/**
 * @param {number[]} nums
 * @return {number}
 */
var pivotIndex = function(nums) {
  const sum = nums.reduce((s, n) => s += n, 0)

  for (let i = 0, leftSum = 0; i < nums.length; i++) {
    if (leftSum === sum - leftSum - nums[i]) return i
    leftSum += nums[i]
  }

  return -1
};