##### ###### ##### ### # # ### # # ###### ## ## ## ## ## ## ## # # # # # ## ##### #### ##### # # # # # # # #### ## # ## ## ## ## # # # # # ## ## # ###### ## ### # ### # ######
##### ###### ##### ### # # ### # # ###### ## ## ## ## ## ## ## # # # # # ## ##### #### ##### # # # # # # # #### ## # ## ## ## ## # # # # # ## ## # ###### ## ### # ### # ######
One or two digits
At each position, you have up to two choices:
Take one digit (if it's -, not )
Take two digits (if they form -) For "": at position , you can take "" (valid) or "" (valid). At position , you can take "" (valid) or "" (valid). These choices branch out into different decodings. This branching, choosing one or two digits, is the core recursive structure of the problem.