Programming Interview Questions 1 : Find the Pair in the Array

Earlier, I wrote about my ways of interviewing job candidates at my work place. For a very long I wanted to post some of the questions that I ask, so that it might be useful to people like me. Hence, here comes the first of the series. I hope the more such posts would come at regular interval  :-)

Question  : Given an array of unsorted integers of size N (a1, a2, a3.... aN) and another integer k. Find the pair of numbers within the array that would sum up to k. To make the problem simple, there is no duplicates of integers in the array.