Given an integer array nums, find the pivot index of the array.

The pivot index is defined as the index where the sum of all elements strictly to its left equals the sum of all elements strictly to its right. If the index lies on the left edge of the array, the left sum is considered $0$ since no elements exist to its left. The same applies when the index lies on the right edge.

Return the leftmost pivot index. If no such index exists, return $-1$.

Constraints:

• $1 \leq$ nums.length $\leq 10^4$ ...