#724 - Find Pivot Index

EasyPrefix Sum

Given an integer array nums, return the leftmost pivot index. The pivot index is where the sum of all elements strictly to the left equals the sum of all elements strictly to the right. If no such index exists, return -1. An element at the edge has 0 on one side.

Input	Output
nums = [1,7,3,6,5,6]	3
nums = [1,2,3]	-1
nums = [2,1,-1]	0

Solution Space

Try it

/**
 * Returns the leftmost pivot index where leftSum === rightSum.
 *
 * @param nums Array of integers
 * @returns    Pivot index, or -1 if none exists
 */
export function pivotIndex(nums: number[]): number {
  const total = nums.reduce((sum, n) => sum + n, 0);
  let leftSum = 0;

  for (let i = 0; i < nums.length; i++) {
    // rightSum = total − leftSum − nums[i]
    if (leftSum === total - leftSum - nums[i]) return i;
    leftSum += nums[i];
  }

  return -1;
}

Approach: prefix sum – compute the total sum once, then iterate while maintaining a running `leftSum`. At each index, `rightSum = total − leftSum − nums[i]`.

Time: O(n) – one pass to compute `total`, one pass to find the pivot.

Space: O(1) – only two numeric variables.

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