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) {
        int len1 =S.length();
        int len2 = T.length();
        int[][] dp = new int [len1+1][len2+1];
        for(int i=0;i<=len2;i++){
            dp[0][i] = 0;
        }
        for(int i=0;i<=len1;i++){
            dp[i][0] = 1;
        }
        for(int i=1;i<=len1;i++){
            for(int j=1;j<=len2;j++){
                if(S.charAt(i-1)!=T.charAt(j-1))
                    dp[i][j] = dp[i-1][j];
                else 
                    dp[i][j] = dp[i-1][j] + dp[i-1][j-1];
            }
        }
        return dp[len1][len2];
    }
}