Distinct Subsequences

public class Solution {    public int numDistinct(String S, String T) {        // Start typing your Java solution below        // DO NOT write main() function        if(S==null||T==null)return 0;        int lent = T.length();        int lens = S.length();        if(lens<lent)return 0;        int[][] dp = new int[lent][lens];        dp[0][0] = S.charAt(0)==T.charAt(0)?1:0;        for(int j = 1; j < lens; j++){            if(S.charAt(j)==T.charAt(0))dp[0][j] = dp[0][j-1]+1;            else dp[0][j] = dp[0][j-1];        }        for(int i = 1; i < lent; i++)            dp[i][0] = 0;        for(int i = 1; i < lent; i++)            for(int j = 1; j < lens; j++){                if(S.charAt(j)==T.charAt(i))dp[i][j] = dp[i-1][j-1] + dp[i][j-1];                else dp[i][j] = dp[i][j-1];            }        return dp[lent-1][lens-1];    }}