non repeating char

1def first_non_repeating_character(str1):
2  char_order = []
3  ctr = {}
4  for c in str1:
5    if c in ctr:
6      ctr[c] += 1
7    else:
8      ctr[c] = 1 
9      char_order.append(c)
10  for c in char_order:
11    if ctr[c] == 1:
12      return c
13  return None
14
15print(first_non_repeating_character('abcdef'))
16print(first_non_repeating_character('abcabcdef'))
17print(first_non_repeating_character('aabbcc'))
18
19