main idea: given total sum, use total-psum[left] - pivot == psum[left]

class Solution:
    def pivotIndex(self, nums: List[int]) -> int:
        # time O(N) space (1)
        # step1: get total sum
        total = sum(nums)
        psum = 0
        # step2: iterate through nums, get psum 
        for i,num in enumerate(nums):
            # step3: check if current number is pivot number  ==> total -psum + current num == psum  
            if total - psum - num == psum:
                return i
            psum += num
        # step4 if none meet pivot standard return -1 
        return -1