Target Indices

Given an array and a target integer, write a function sum_pair_indices that returns the indices of two integers in the array that add up to the target integer. If not found, just return an empty list.

Note: Can you do it on $O(n)$ time?

Note: Even though there could be many solutions, only one needs to be returned.

Example 1:

Input:

array = [1 2 3 4] 
target = 5 

Output:

def sum_pair_indices(array, target) -> [0 3] or [1 2]

Example 2:

Input:

array = [3]
target = 6 

Output:

Do NOT return [0 0] as you can't use an index twice.