leetcode--Distinct Subsequences

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"

Return 3.

public class Solution {
    public int numDistinct(String S, String T) {
       //special case
		int slen = S.length(), tlen = T.length();
		if(slen == 0 || tlen == 0 || slen < tlen) 
			return 0;
		//dynamic programming
		int[][] num = new int[slen + 1][tlen + 1];
		for(int i = 0; i < slen + 1; ++i)
			num[i][0] = 1;  //take nothing from target string T.
		for(int i = 1; i < tlen + 1; ++i)
			num[0][i] = 0;  //take nothing from source string S.
		
		for(int i = 1; i < slen + 1; ++i) {
			for(int j = 1; j < tlen + 1; ++j) {
				if(S.charAt(i - 1) == T.charAt(j - 1))
					num[i][j] = num[i - 1][j - 1] + num[i - 1][j];
				else
					num[i][j] = num[i - 1][j];
			}
		}
		return num[slen][tlen];   
    }
}