1068. 寻找数组的中心索引

class Solution:
    """
    @param nums: an array
    @return: the "pivot" index of this array
    """
    ‘‘‘
    大致思路:
    1.给定初始值，left，right = 0,sum(nums),一次求和即可，不用多次求和。双指针的方法，left += nums[index],right -= nums[index]
    2.如果left == right ,则直接返回index,否则-1
    ‘‘‘
    def pivotIndex(self,nums):
        left,right = 0,sum(nums)
        for index in range(len(nums)):
            right -= nums[index] 
            if left == right:
                return index
            left += nums[index]
        return -1