Distinct Subsequences

public class Solution {
    public int numDistinct(String S, String T) {
        // IMPORTANT: Please reset any member data you declared, as
        // the same Solution instance will be reused for each test case.
        if(S == null || S.length() == 0){
            return 0;
        }
        
        if(T == null || T.length() == 0){
            return 1;
        }
        
        int sLen = S.length(), tLen = T.length();
        if(sLen < tLen){
            return 0;
        }
        
        int[][] dp = new int[sLen + 1][tLen + 1];
        for(int i = 0; i < sLen + 1; i++){
            dp[i][0] = 1;
        }
        
        for(int i = 1; i < tLen + 1; i++){
            dp[0][i] = 0;
        }
        
        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)){
                    dp[i][j] = dp[i - 1][j - 1] + dp[i - 1][j];
                } else {
                    dp[i][j] = dp[i - 1][j];
                }
            }
        }
        
        return dp[sLen][tLen];
        
    }
}