Given an array of integers, find two indices that split the array into three contiguous parts with equal sums. Return the indices or -1 if it is impossible.

First compute the total sum of the array. If it is not divisible by 3, return -1. Let target = total / 3. Iterate through the array while maintaining a running sum. When the running sum equals target for the first time, record that index as the end of the first part. Continue iterating; when the running sum equals 2 * target, record that index as the end of the second part. Ensure that the second index is not the last element, because the third part must contain at least one element. If both indices are found, return them; otherwise return -1. This algorithm runs in O(n) time and O(1) extra space.