This program illustrates the use of the STL stable_sort() algorithm (extended
version) to sort a vector of integers, when a first integer comes before a
second if and only if the first has a smaller digit sum than the second.

The sort is stable, in that duplicate values retain their original order.

An interesting exercise is to change the call to stable_sort() to a call
to sort() and observe the difference in output, if any.
Press Enter to continue ...

Here are the original contents of the vector:
17 12 14 111 19 23 15 61 20 81 11 21 41 34
Press Enter to continue ...

And here the sorted contents of the vector:
20 11 12 111 21 14 23 41 15 61 34 17 81 19
Press Enter to continue ...