'''
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 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.

 

Example 1:
Input: nums = [1,7,3,6,5,6]
Output: 3
Explanation:
The pivot index is 3.
Left sum = nums[0] + nums[1] + nums[2] = 1 + 7 + 3 = 11
Right sum = nums[4] + nums[5] = 5 + 6 = 11


Example 2:
Input: nums = [1,2,3]
Output: -1
Explanation:
There is no index that satisfies the conditions in the problem statement.

Example 3:
Input: nums = [2,1,-1]
Output: 0
Explanation:
The pivot index is 0.
Left sum = 0 (no elements to the left of index 0)
Right sum = nums[1] + nums[2] = 1 + -1 = 0
'''

class Solution(object):
    def pivotIndex(self, nums):
        left_sum = 0
        for idx, elem in enumerate(nums):
            if left_sum == (sum(nums) - left_sum - elem):
                return idx
            left_sum += elem
        return -1
# BALANCE WEIGHT O(N) TC
class Solution(object):
    def pivotIndex(self, nums):
        # Initialization:
        # Left hand side be empty, and
        # Right hand side holds all weights.
        total_weight_on_left, total_weight_on_right = 0, sum(nums)
        for idx, current_weight in enumerate(nums):
            total_weight_on_right -= current_weight
            if total_weight_on_left == total_weight_on_right:
                # balance is met on both sides
                # i.e., sum( nums[ :idx] ) == sum( nums[idx+1: ] )
                return idx
            total_weight_on_left += current_weight
        return -1
