public int numDecodings(String s) {
    int len = 0;
    if (s == null || (len = s.length()) == 0) {
        return len;
    }
    char[] content = s.toCharArray();
    int[] dp = new int[len];
    // start with '0' is impossible and return 0;
    if (content[0] == '0') {
        return 0;
    }

    // dp[i] means the max decoration ways up to i-th index.
    dp[0] = 1;
    for (int i = 1; i <= len - 1; i++) {
        char chNow = content[i];
        char chBefore = content[i - 1];
        // current char is 0, so it can't stand alone.
        if (chNow == '0') {
            // can't combine with former, return 0
            if (chBefore != '1' && chBefore != '2') {
                return 0;
            } else {
                // combine with former,so dp[i]= dp[i-2] (except for i==1)
                dp[i] = (i == 1 ? 1 : dp[i - 2]);
            }
        } else {
            // can stand alone or combine with former
            if (chBefore == '1' || (chBefore == '2' && chNow <= '6')) {
                dp[i] = dp[i - 1] + (i == 1 ? 1 : dp[i - 2]);
            } else {
                // only can stand alone.
                dp[i] = dp[i - 1];
            }
        }
    }
    return dp[len - 1];
}