Find the Balance Index

Return the **leftmost** index `i` such that the sum of all elements strictly to the left of `i` equals the sum of all elements strictly to its right. An empty side sums to `0`. If no such index exists, return `-1`. **Example 1:** ``` Input: nums = [1, 7, 3, 6, 5, 6] Output: 3 ``` **Example 2:** ``` Input: nums = [1, 2, 3] Output: -1 ``` **Example 3:** ``` Input: nums = [2, 1, -1] Output: 0 ```