Find Pivot Index
Easy· running sum
Problem
Return the leftmost index where the sum of all elements strictly to its left equals the sum of all elements strictly to its right. If no such index exists, return -1. An empty side counts as 0.
Examples
Input: nums = [1,7,3,6,5,6]
Output: 3
Left of index 3: 1 + 7 + 3 = 11. Right of it: 5 + 6 = 11.
Input: nums = [1,2,3]
Output: -1
Constraints
- • 1 <= nums.length <= 10^4
- • -1000 <= nums[i] <= 1000
Hints & approach
Hint 1
If you know the total and the sum to the left of i, you can derive the right sum.
Hint 2
right = total - left - nums[i].
Approachtry the hints first
Compute the total sum once. Walk the array with a running left sum; at each index i, the right sum is total - left - nums[i]. Return the first i where the two are equal, then add nums[i] to the left sum before moving on. Return -1 if no index qualifies.
Time O(n) · Space O(1)