Find Pivot Index (Prefix Sum Made Simple)

The Pivot Index of an array is the index where: Sum of elements on the left = Sum of elements on the right If no such index exists, return -1. Key Idea 1) Instead of calculating left and right sums again and again (which is slow), we can 2) Compute the total sum of the array. 3) Keep a running left sum. At each index: -> Right sum = totalSum - leftSum - nums[i] -> If leftSum == rightSum, that index is the pivot. This avoids nested loops and makes the solution efficient. Efficient Approach (Prefix Logic) Steps: Find total sum. Traverse the array. At each index: Check if left sum equals right sum. Update left sum. C++ Implementation class Solution { public: int pivotIndex(vector<int>& nums) { int totalSum = 0; for (int num : nums) totalSum += num; int leftSum = 0; for (int i = 0; i < nums.size(); i++) { if (leftSum == totalSum - leftSum - nums[i]) return i; leftSum += nums[i]; } return -1; } }; Example Input: [1, 7, 3, 6, 5, 6] At index 3: -> Left sum = 1 + 7 + 3 = 11 . -> Right sum = 5