input: Write a python function to find all permutations of a given string.
  assert permutations("abc") == ["abc", "acb", "bac", "bca", "cab", "cba"]
  assert permutations("ab") == ["ab", "ba"]
  assert permutations("a") == ["a"]
output: def permutations(s):
      if len(s) == 1:
          return [s]
      perm_list = []
      for i in range(len(s)):
          part = s[i]
          remaining = s[:i] + s[i+1:]
          for p in permutations(remaining):
              perm_list.append(part + p)
      return perm_list