How to write an algorithm to check if the sum of any two numbers in an array/list matches a given number?

Question

How can I write an algorithm to check if the sum of any two numbers in an array/list matches a given number with a complexity of nlogn?

Answer 1

I'm sure there's a better way, but here's an idea:

1. Sort array
2. For every element e in the array, binary search for the complement (sum - e)

Both these operations are O(n log n).