public int numDistinct(String s, String t) {

        int n1 = s.length();

        int n2 = t.length();

        int[][] dp = new int[n2 + 1][n1 + 1];

        for (int i = 0; i <= n1; i++) dp[0][i] = 1;

        for (int i = 1; i <= n2; i++) {

            for (int j = 1; j <= n1; j++) {

                if (t.charAt(i - 1) == s.charAt(j - 1)) dp[i][j] = dp[i - 1][j - 1] + dp[i][j - 1];

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

            }

        }

        return dp[n2][n1];

    }