input: Write a python function to compare two strings and check if they differ by exactly one character.
  assert compare_strings('apple', 'appla') == True
  assert compare_strings('apple', 'apale') == False
  assert compare_strings('apple', 'apple') == False
output: def compare_strings(s1, s2):
      if len(s1) != len(s2):
          return False
      
      diff_count = 0
      for c1, c2 in zip(s1, s2):
          if c1 != c2:
              diff_count += 1
          if diff_count > 1:
              return False
              
      return diff_count == 1