output_description: Assume that there are K strings of length N that are in normal form: w_1,
  \ldots, w_K in lexicographical order. Output should be in the following
  format:
  
      
      
      w_1
      :
      w_K
      
  
  * * *
input_description: Input is given from Standard Input in the following format:
  
      
      
      N
attempt: n = int(input())
  c = "abcdefghijk"
  d = [[0]]
  for i in range(n - 1):
      pd, d = d, []
      for j in pd:
          m = max(j)
          for k in range(m + 2):
              d.append(j + [k])
  for i in d:
      for j in i[:-1]:
          print(c[j], end="")
      print(c[i[-1]])
  
problem_description: Statement
  
  In this problem, we only consider strings consisting of lowercase English
  letters.
  
  Strings s and t are said to be **isomorphic** when the following conditions
  are satisfied:
  
    * |s| = |t| holds.
    * For every pair i, j, one of the following holds:
      * s_i = s_j and t_i = t_j.
      * s_i \neq s_j and t_i \neq t_j.
  
  For example, `abcac` and `zyxzx` are isomorphic, while `abcac` and `ppppp` are
  not.
  
  A string s is said to be in **normal form** when the following condition is
  satisfied:
  
    * For every string t that is isomorphic to s, s \leq t holds. Here \leq denotes lexicographic comparison.
  
  For example, `abcac` is in normal form, but `zyxzx` is not since it is
  isomorphic to `abcac`, which is lexicographically smaller than `zyxzx`.
  
  You are given an integer N. Print all strings of length N that are in normal
  form, in lexicographically ascending order.
samples: [{"input": "1", "output": "a\n    \n\n* * *"}, {"input": "2", "output": "aa\n    ab"}]