Write a non-recursive program to store all possible strings of length 'k' in an array, that can be formed from a set of 'n' characters in C language. The program must NOT be recursive.

For example,

Given n = 4, k = 3.

Set of characters = {'A', 'C', 'G', 'T'}

The output array should store all the 3 letter combinations of these. There will be 64 combinations.

AAA

AAC

AAT

AAG

....... (total 64 combinations)