Given an integer array `nums`, partition it into two (contiguous) subarrays `left` and `right` so that:

- Every element in `left` is less than or equal to every element in `right`.
- `left` and `right` are non-empty.
- `left` has the smallest possible size.

Return the length of `left` after such a partitioning.

Test cases are generated such that partitioning exists.

Example 1:

    Input: nums = [5,0,3,8,6]
    Output: 3
    Explanation: left = [5,0,3], right = [8,6]

Example 2:

    Input: nums = [1,1,1,0,6,12]
    Output: 4
    Explanation: left = [1,1,1,0], right = [6,12]

Constraints:

- `2 <= nums.length <= 105`
- `0 <= nums[i] <= 106`
- There is at least one valid answer for the given input.