【LeetCode】724. Find Pivot Index

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class Solution:
    # 基本想法，遍历数组，但是 Time Limit Exceeded
    def pivotIndex(self, nums):
        """
        :type nums: List[int]
        :rtype: int
        """
        if not nums:
            return -1
        for i in range(len(nums)):
            if sum(nums[:i]) == sum(nums[i+1:]):
                return i
        return -1



class Solution:
    # 只计算数组和的一半，节省计算资源
    def pivotIndex(self, nums):
        """
        :type nums: List[int]
        :rtype: int
        """
        size = sum(nums)
        left = 0
        for i in range(len(nums)):
            if left * 2 + nums[i] == size:
                return i
            left += nums[i]
        return -1