Find Pivot Index

01 · Question

Given an array of integers nums, return the pivot index where the sum of the numbers to the left equals the sum of the numbers to the right.

02 · Analysis

Compute total once. At index i, the right sum is total-left-nums[i]. Compare before adding nums[i] to left, so the pivot belongs to neither side. Return the first match; return -1 if none exists. Empty sides have sum zero. Time O(n), auxiliary space O(1).

03 · Solution

Reference solution

1def pivotIndex(nums: List[int]) -> int:
2    total = sum(nums)
3    left = 0
4
5    for i, x in enumerate(nums):
6        if left == total - left - x:
7            return i
8        left += x
9
10    return -1

Subarray Sum Equals K

Batch Range Additions