input: Write a python function to print all combinations of a given string.
  assert printAllCombinations("abc") == None # Output should be "abc" "bac" "cba"
  assert printAllCombinations("xyz") == None # Output should be "xyz" "yxz" "zyx"
  assert printAllCombinations("123") == None # Output should be "123" "213" "321"
output: def printAllCombinations(s):
      def helper(prefix, remaining):
          if len(remaining) == 0:
              print(prefix)
          else:
              for i in range(len(remaining)):
                  helper(prefix + remaining[i], remaining[:i] + remaining[i+1:])
      
      helper("", s)