The problem statement is, an long array is given with `n` elements, we need to find all the pairs of numbers in this long array which have a constant difference `k`. I will post three methods. The first method is the brute force method and has a runtime complexity `O (n ^{2})`, the next method has runtime complexity

`O (n*log (n))`, and the last one will have the runtime complexity

`O (n)`.