text: __________________________________________________________________________________________________
  class Solution:
      def maximum69Number (self, num: int) -> int:
          for idx in range(len(str(num))):
              if str(num)[idx]=='6':
                  return int(str(num)[:idx]+'9'+str(num)[idx+1:])
          return num
  __________________________________________________________________________________________________
  sample 24 ms submission
  class Solution:
      def maximum69Number (self, num: int) -> int:
          num_str = list(str(num))
          for i, ch in enumerate(num_str):
              if ch == '6':
                  num_str[i] = '9'
                  break
          return int(''.join(num_str))
  __________________________________________________________________________________________________
  