code: def _getCommonStart(self, left, right):
          
          prefix = []
          for a, b in zip(left, right):
              if a == b:
                  prefix.append(a)
              else:
                  break
  
          return ''.join(prefix)
docs: Return the common prefix of the 2 strings.
  
          @param left: one string
          @param right: another string