Write a program to find the amount to be paid to or taken from the other friends who paid at the moments?

Question

Write a program which finds all pairs of elements in an array whose sum is equal to a given number

Input  array: [-2,1,3,5,6,7,8,10,12,15,17,19,20]
Sum : 20

Output

6, 12
3, 15
1, 17
-2, 20
8, 10

Required best time complexity and effective code