input: Write a python function to generate all permutations of a string.
  assert permutation("ABC") == ['ABC', 'ACB', 'BAC', 'BCA', 'CAB', 'CBA']
  assert permutation("AB") == ['AB', 'BA']
  assert permutation("A") == ['A']
output: def permutation(s):
      if len(s) == 1:
          return [s]
      
      perms = []
      for i, char in enumerate(s):
          for perm in permutation(s[:i] + s[i+1:]):
              perms.append(char + perm)
      
      return perms