Find Pivot Index

Problem

Given an array of integers nums, calculate the pivot index of this array. The pivot index is the index where the sum of all the numbers strictly to the left of the index is equal to the sum of all the numbers strictly to the right of the index. Return the leftmost pivot index. If no such index exists, return -1.

Examples

The brute-force approach

For each index i, compute the sum of elements to the left and right separately. Return i if they match.

for i in range(n):
    left = sum(nums[:i])
    right = sum(nums[i+1:])
    if left == right: return i
return -1

O(n²) — recomputing left and right sums from scratch at each index.

Spotting the pattern

This is a Prefix Sum problem. The key question to ask yourself:

If you know the total sum and the running left sum, what formula gives you the right sum without scanning the array again?

Answering that is where it clicks, and it's exactly what the guided walkthrough below builds with you: the pattern reasoning, a progressive hint ladder that never spoils the answer, a row-by-row dry run, the optimized solution, and an in-browser editor to run your code against real test cases.