LeetCode 115 Distinct Subsequences - Walkthrough

Count how many times $t$ appears as a subsequence of $s$. Let $dp[i][j]$ = count for $s[0..i-1]$ and $t[0..j-1]$. If $s[i-1] = t[j-1]$: we can match this character ($dp[i-1][j-1]$) or skip it ($dp[i-1][j]$). Total: $dp[i][j] = dp[i-1][j-1] + dp[i-1][j]$. If they don't match: must skip $s[i-1]$. $dp[i][j] = dp[i-1][j]$. Example: $s = \text{"rabbbit"}$, $t = \text{"rabbit"}$.

Answer: $3$ (three ways to choose which 'b' to skip).