LeetCode 91 Decode Ways - The Choices

At each position, you have up to two choices:

$1.$ Take one digit (if it's $1$-$9$, not $0$)

$2.$ Take two digits (if they form $10$-$26$) For "$226$": at position $0$, you can take "$2$" (valid) or "$22$" (valid). At position $1$, you can take "$2$" (valid) or "$26$" (valid). These choices branch out into different decodings. This branching, choosing one or two digits, is the core recursive structure of the problem.