Task

Write a function that accepts msg string and returns local tops of string from the highest to the lowest.
The string's tops are from displaying the string in the below way:

                                                      3
                              p                     2   4
            g               o   q                 1
  b       f   h           n       r             z
a   c   e       i       m          s          y
      d           j   l             t       x
                    k                 u   w
                                        v

The next top is always 1 character higher than the previous one. For the above example, the solution for the abcdefghijklmnopqrstuvwxyz1234 input string is 3pgb.

• When the msg string is empty, return an empty string.
• The input strings may be very long. Make sure your solution has good performance.

Check the test cases for more samples.