We'll say that 2 strings "match" if they are non-empty and their first chars are the same. Loop over and then return the given array of non-empty strings as follows: if a string matches an earlier string in the array, swap the 2 strings in the array. A particular first char can only cause 1 swap, so once a char has caused a swap, its later swaps are disabled. Using a map, this can be solved making just one pass over the array. More difficult than it looks.