code: def diff_commonPrefix(self, text1, text2):
      
      # Quick check for common null cases.
      if not text1 or not text2 or text1[0] != text2[0]:
        return 0
      # Binary search.
      # Performance analysis: https://neil.fraser.name/news/2007/10/09/
      pointermin = 0
      pointermax = min(len(text1), len(text2))
      pointermid = pointermax
      pointerstart = 0
      while pointermin < pointermid:
        if text1[pointerstart:pointermid] == text2[pointerstart:pointermid]:
          pointermin = pointermid
          pointerstart = pointermin
        else:
          pointermax = pointermid
        pointermid = (pointermax - pointermin) // 2 + pointermin
      return pointermid
docs: Determine the common prefix of two strings.
  
      Args:
        text1: First string.
        text2: Second string.
  
      Returns:
        The number of characters common to the start of each string.