Split an array into two halves with equal sums

You are given an array nums of positive integers. Find an index i that splits it into two non-empty contiguous parts with the same sum:

• the left part is nums[0..i] (it includes nums[i]),
• the right part is nums[i+1..n-1].

Return i, or -1 if no such index exists.

def find_split_index(nums: list[int]) -> int: ...

Constraints

• 0 <= len(nums) <= 10^5
• 1 <= nums[k] <= 10^9
• Both parts must contain at least one element.

Example 1

nums = [4, 2, 3, 3]
output: 1

Left [4, 2] sums to 6 and right [3, 3] sums to 6.

Example 2

nums = [1, 2, 5]
output: -1

The total is 8, but no prefix sums to 4 (the prefixes are 1, 3, 8).

Aim for O(n) time and O(1) extra space.

Reconstructed from the reported question; details may differ from what interviewers ask.