We'll say that 2 strings "match" if they are non-empty and their first chars are the same. Loop over and return the given array of non-empty strings as follows: 1- If a string matches an earlier string in the array, swap the 2 strings. 2- A particular first char can only cause 1 swap, so once a char has caused a swap, its later swaps are disabled. This can be solved in one pass achieving Θ(n) amortized time only by using a HashMap and HashSet. So you MUST use HashMap and HashSet in your solution.