public boolean isSubsequence2(String s, String t) {
        if (s == null || t == null) return false;
        boolean[][] dp = new boolean[s.length() + 1][t.length() + 1];
        for (int j = 0; j <= t.length(); ++j) dp[0][j] = true;
        for (int i = 1; i <= s.length(); ++i) {
            for (int j = 1; j <= t.length(); ++j) {
                if (s.charAt(i - 1) != t.charAt(j - 1)) {
                    dp[i][j] = dp[i][j - 1];
                } else {
                    dp[i][j] = dp[i][j - 1] || dp[i - 1][j - 1];
                }
            }
        }
        return dp[s.length()][t.length()];
    }