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