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`

Note: This question is the same as 724: https://leetcode.com/problems/find-pivot-index/