LeetCode: Find Pivot Index Solution

2 prefix sums

Approach

Create two prefix sums:

One for accumulated sum from left to right
One for accumulated sum from right to left

Might reverse right-to-left one to be easier in later comparision (avoid i versus n - i - 1)

Implementation

1var pivotIndex = function (nums) {
2  const n = nums.length
3  let [leftSum, rightSum] = [0, 0]
4  const leftSums = []
5  const rightSums = []
6
7  for (let i = 0; i < n; i++) {
8    leftSum += nums[i - 1] || 0
9    leftSums.push(leftSum)
10  }
11
12  for (let i = n - 1; i >= 0; i--) {
13    rightSum += nums[i + 1] || 0
14    rightSums.push(rightSum)
15  }
16  rightSums.reverse()
17
18  let res = -1
19
20  for (let i = 0; i < n; i++) {
21    if (leftSums[i] === rightSums[i]) {
22      res = i
23      break
24    }
25  }
26
27  return res
28}