public int numDistinct(String s, String t) {

int n = s.length(), m = t.length();

int[][] dp = new int[m+1][n+1];

for(int j = 0; j < n; j++){

dp[0][j] = 1;

}

for(int i = 1; i < m+1; i++){

for(int j = 1; j < n+1; j++){

if(s.charAt(j-1)==t.charAt(i-1)){

dp[i][j] = dp[i-1][j-1]+dp[i][j-1];

} else {

dp[i][j] = dp[i][j-1];

}

}

}

return dp[m][n];

}

public static void main(String[] args) {

DistinctSubsequences test = new DistinctSubsequences();

String s = "ab";

String t = "";

System.out.println(test.numDistinct(s, t));

}