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

def sum_in(numbers, sum_):
    """whether any two numbers from `numbers` form `sum_`."""
    a = set(numbers) # O(n)
    return any((sum_ - n) in a for n in a) # O(n)

Example:

>>> sum_in([200, -10, -100], 100)
True