More Practice

HashMaps and HashSets are useful in a wide variety of challenges. Below is another practice challenge.

Challenge

Given a list of numbers, can you find all the pairs of numbers whose sum equals k?

Input
The provided boilerplate takes 2 parameters: an array ar, and the special number k. This is the raw input format:

The first line of input will contain t. t test cases follow, with each case consisting of n, followed by a line with n numbers. The first number on that line is k, and the remaining numbers are the list.

Output
Print the pairs of numbers. Print each pair with the first number followed by the second, and print all the pairs in the order that the second number appears in the list.

Sample Input/Output
Input

1
8
12 1 9 11 13 2 3 7

1 11 9 3 

12 is the special number. There are 2 pairs with that sum: 1 & 11 and 9 & 3. 11 appears before 3 in the input, so that pair is printed first.

Full Credit
Solve the problem with only one iteration through the numbers.