LeetCode 91 Decode Ways - State Definition

Define $dp[i]$ = number of ways to decode the substring from index $i$ to the end. Base case: $dp[n] = 1$ (empty string = one valid decoding: do nothing). The answer is $dp[0]$, the ways to decode the entire string.

You could also define it as "ways to decode first $i$ characters" and build forward. Both work. Choosing the right state is often the hardest part. The state must capture everything needed to solve the subproblem independently.