leetcode/724. Find Pivot Index.py at master · albertalexandrov/leetcode · GitHub

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"""
https://leetcode.com/problems/find-pivot-index/

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):
    """
    Time: O(n)
    Space: O(1)

    """
    def pivotIndex(self, nums):
        """
        :type nums: List[int]
        :rtype: int
        """
        left_sum, right_sum = 0, sum(nums)

        for idx, num in enumerate(nums):
            right_sum -= num

            if left_sum == right_sum:
                return idx

            left_sum += num

        return -1


solution = Solution()

assert solution.pivotIndex([1, 7, 3, 6, 5, 6]) == 3
assert solution.pivotIndex([1, 2, 3]) == -1
assert solution.pivotIndex([2, 1, -1]) == 0