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];
    }
}