input: Write a python function to check if two strings are permutations of each other.
  assert is_permutation("abc", "bca") == True
  assert is_permutation("abc", "abcd") == False
  assert is_permutation("aabbcc", "baccab") == True
output: def is_permutation(s1, s2):
      return sorted(s1) == sorted(s2)