1991-Find-the-Middle-Index-in-Array.py

'''
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. 

Constraints:
1 <= nums.length <= 100
-1000 <= nums[i] <= 1000
'''

class Solution:
    def findMiddleIndex(self, nums: List[int]) -> int:
        left_sum = 0
        right_sum = sum(nums)
        
        for i, num in enumerate(nums):
            if left_sum == right_sum - num:
                return i
            
            left_sum += num
            right_sum -= num
            
        return -1