LeetCode 91 Decode Ways - Problem Statement

A message of letters A-Z is represented as numbers: $A=1$, $B=2$, ..., $Z=26$. Given a string of digits like "$226$", count how many ways you can decode it. "$226$" can be "BZ" ($2$,$26$), "VF" ($22$,$6$), or "BBF" ($2$,$2$,$6$).

That's $3$ ways. Before reading on, think: at each position, what choices do you have? And when is a choice invalid? Constraints: $n \le 100$. Watch for leading zeros: '06' is not a valid encoding.