Find Pivot Index
The key idea
The right sum is the total minus the left sum minus the current element. Carry a running left sum as you scan; at each index check if leftSum equals total - leftSum - nums[i]. No nested loop is needed once you precompute the total.
Problem
Given an array of integers nums, calculate the pivot index of this array.
The pivot index is an 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 index's right.
If the index is on the left edge of the array, then the left sum is 0 because there are no elements to the left. This also applies to the right edge of the array.
Return the leftmost pivot index. If no such index exists, return -1.
Constraints
- 1 <= nums.length <= 10^4
- -1000 <= nums[i] <= 1000
Examples
Input: nums = [1,7,3,6,5,6]
Output: 3
Input: nums = [1,2,3]
Output: -1
Input: nums = [2,1,-1]
Output: 0
Complexity
Time: O(n) Space: O(1)
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