distinct-subsequences Java code

Given a string S and a string T, count the number of distinct subsequences of T in S.

A subsequence of a string is a new string which is formed from the original string by deleting some (can be none) of the characters without disturbing the relative positions of the remaining characters. (ie,”ACE”is a subsequence of”ABCDE”while”AEC”is not).

Here is an example:

S =”rabbbit”, T =”rabbit”

Return3.

public class Solution {
public int numDistinct(String S, String T) {
if(S == null || T == null || T.length() == 0) return 0;
int n = T.length();
int[] dp = new int
;
for(int i = 0; i < S.length(); i++)
for(int j = n - 1; j >= 0; j--)
if(S.charAt(i) == T.charAt(j))
dp[j] = (j == 0 ? 1 : dp[j-1]) + dp[j];
return dp[n-1];
}
}