1991. Find the Middle Index in Array.py

"""
https://leetcode.com/problems/find-the-middle-index-in-array/

Given a 0-indexed integer array nums, find the leftmost middleIndex (i.e., the smallest amongst all the possible ones).

A middleIndex is an index where
nums[0] + nums[1] + ... + nums[middleIndex-1] == nums[middleIndex+1] + nums[middleIndex+2] + ... + nums[nums.length-1].

If middleIndex == 0, the left side sum is considered to be 0. Similarly, if middleIndex == nums.length - 1,
the right side sum is considered to be 0.

Return the leftmost middleIndex that satisfies the condition, or -1 if there is no such index.

Example 1:
    Input: nums = [2,3,-1,8,4]
    Output: 3
    Explanation: The sum of the numbers before index 3 is: 2 + 3 + -1 = 4
    The sum of the numbers after index 3 is: 4 = 4

Example 2:
    Input: nums = [1,-1,4]
    Output: 2
    Explanation: The sum of the numbers before index 2 is: 1 + -1 = 0
    The sum of the numbers after index 2 is: 0

Example 3:
    Input: nums = [2,5]
    Output: -1
    Explanation: There is no valid middleIndex.

"""


class Solution(object):
    def findMiddleIndex(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.findMiddleIndex([2, 3, -1, 8, 4]) == 3
assert solution.findMiddleIndex([1, -1, 4]) == 2
assert solution.findMiddleIndex([2, 5]) == -1