Write a method that swaps the first and last 'swapNum' number of characters of the input parameter string 'str'. For example, if str="swap these" and swapNum=2 you would return "seap thesw" (because you swapped the first and last two characters. As a precondition, you may assume that swapNum will be less than or equal to half the length of str and that str will have at least two characters.