public int numDistinct(String S, String T) {
        int[][] dp = new int[T.length() + 1][S.length() + 1];
        dp[0][0] = 1;
        //first column all 0
        for (int i = 1; i <= T.length(); i++) {
            dp[i][0] = 0;
        }
        //first row all 1
        for (int j = 1; j <= S.length(); j++) {
            dp[0][j] = 1;
        }
        //find match, then +1
        for (int i = 1; i <= T.length(); i++) {
            for (int j = 1; j <= S.length(); j++) {
                dp[i][j] = dp[i][j - 1];
                if (T.charAt(i - 1) == S.charAt(j - 1)) {
                    dp[i][j] += dp[i - 1][j - 1];
                }
            }
        }
        //return the number of last postion
        return dp[T.length()][S.length()];
    }