591 Tag Validator

Given a string representing a code snippet, you need to implement a tag validator to parse the code and return whether it is valid. A code snippet is valid if all the following rules hold:

1. The code must be wrapped in a valid closed tag. Otherwise, the code is invalid.
2. A closed tag (not necessarily valid) has exactly the following format : <TAG_NAME>TAG_CONTENT</TAG_NAME>. Among them, <TAG_NAME> is the start tag, and </TAG_NAME> is the end tag. The TAG_NAME in start and end tags should be the same. A closed tag is valid if and only if the TAG_NAME and TAG_CONTENT are valid.
3. A valid TAG_NAME only contain upper-case letters, and has length in range [1,9]. Otherwise, the TAG_NAME is invalid.
4. A valid TAG_CONTENT may contain other valid closed tags, cdata and any characters (see note1) EXCEPT unmatched <, unmatched start and end tag, and unmatched or closed tags with invalid TAG_NAME. Otherwise, the TAG_CONTENT is invalid.
5. A start tag is unmatched if no end tag exists with the same TAG_NAME, and vice versa. However, you also need to consider the issue of unbalanced when tags are nested.
6. A < is unmatched if you cannot find a subsequent >. And when you find a < or </, all the subsequent characters until the next > should be parsed as TAG_NAME (not necessarily valid).
7. The cdata has the following format : <![CDATA[CDATA_CONTENT]]>. The range of CDATA_CONTENT is defined as the characters between <![CDATA[ and the first subsequent ]]>.
8. CDATA_CONTENT may contain any characters. The function of cdata is to forbid the validator to parse CDATA_CONTENT, so even it has some characters that can be parsed as tag (no matter valid or invalid), you should treat it as regular characters.

Valid Code Examples:

Input: "<DIV>This is the first line <![CDATA[<div>]]></DIV>"

Output: True

Explanation: 

The code is wrapped in a closed tag : <DIV> and </DIV>. 

The TAG_NAME is valid, the TAG_CONTENT consists of some characters and cdata. 

Although CDATA_CONTENT has unmatched start tag with invalid TAG_NAME, it should be considered as plain text, not parsed as tag.

So TAG_CONTENT is valid, and then the code is valid. Thus return true.


Input: "<DIV>>>  ![cdata[]] <![CDATA[<div>]>]]>]]>>]</DIV>"

Output: True

Explanation:

We first separate the code into : start_tag|tag_content|end_tag.

start_tag -> "<DIV>"

end_tag -> "</DIV>"

tag_content could also be separated into : text1|cdata|text2.

text1 -> ">>  ![cdata[]] "

cdata -> "<![CDATA[<div>]>]]>", where the CDATA_CONTENT is "<div>]>"

text2 -> "]]>>]"


The reason why start_tag is NOT "<DIV>>>" is because of the rule 6.
The reason why cdata is NOT "<![CDATA[<div>]>]]>]]>" is because of the rule 7.

Invalid Code Examples:

Input: "<A>  <B> </A>   </B>"
Output: False
Explanation: Unbalanced. If "<A>" is closed, then "<B>" must be unmatched, and vice versa.

Input: "<DIV>  div tag is not closed  <DIV>"
Output: False

Input: "<DIV>  unmatched <  </DIV>"
Output: False

Input: "<DIV> closed tags with invalid tag name  <b>123</b> </DIV>"
Output: False

Input: "<DIV> unmatched tags with invalid tag name  </1234567890> and <CDATA[[]]>  </DIV>"
Output: False

Input: "<DIV>  unmatched start tag <B>  and unmatched end tag </C>  </DIV>"
Output: False

Note:

1. For simplicity, you could assume the input code (including the any characters mentioned above) only contain letters, digits, '<','>','/','!','[',']' and ' '.

首先对于这种成对匹配的问题肯定是要用栈stack的，就像之前的匹配括号的问题。那么我们来遍历code字符串，首先是判断，如果当前遍历到的字符非首字符，并且栈为空，那么直接返回false。虽然只是短短的一句但其实非常的重要，这句就排除了很多错误情况，比如开头结尾不是标签的情况，以及没有标签的情况，和开头的标签在中间就闭合了情况等等，非常powerful的一句判断。然后我们来处理包含CDATA的情况，当然是要先匹配到"<![CDATA["，然后我们用find来找结束标志"]]>"，如果没找到，直接返回false，找到了点话就继续遍历，顺便把当前遍历的位置移到结束标志符的最后一位上。

如果我们只匹配到了"</"，说明是个结束标签，那么我们用find来找到右尖括号'>'，如果没找到直接返回false，找到了就把tag到内容提出来，然后看此时的stack，如果stack为空，或者栈顶元素不等于tag，直接返回false，否则就将栈顶元素取出。

如果我们只匹配到了"<"，说明是个起始标签，还是要找右尖括号，如果找不到，或者标签的长度为0，或者大于9了，直接返回true。然后遍历标签的每一位，如果不全是大些字母，返回false，否则就把tag压入栈。那么你可能会有疑问，为啥在处理结束标签时，没有这些额外的判断呢，因为结束标签要和栈顶元素比较，栈里的标签肯定都是合法的，所以如果结束标签不合法，那么肯定不相等，也就直接返回false了。最后我们看栈是否为空，如果不为空，说明有未封闭的标签，返回false。

class Solution {
    public static boolean isValid(String code) {
        Stack<String> s = new Stack<>();
        for (int i = 0; i < code.length();) {
            if (i > 0 && s.isEmpty()) {
                return false;
            }
            if (code.startsWith("<![CDATA[", i)) {
                int j = i + 9;
                i = code.indexOf("]]>", j);
                if (i < 0) {
                    return false;
                }
                i += 3;
            } else if (code.startsWith("</", i)) {
                int j = i + 2;
                i = code.indexOf('>', j);
                if (i < 0 || i == j || i - j > 9) {
                    return false;
                }
                for (int k = j; k < i; k++) {
                    if (!Character.isUpperCase(code.charAt(k))) {
                        return false;
                    }
                }
                String sub = code.substring(j, i++);
                if (s.isEmpty() || !s.pop().equals(sub)) {
                    return false;
                }
            } else if (code.startsWith("<", i)) {
                int j = i + 1;
                i = code.indexOf('>', j);
                if (i < 0 || i == j || i - j > 9) {
                    return false;
                }
                for (int k = j; k < i; k++) {
                    if (!Character.isUpperCase(code.charAt(k))) {
                        return false;
                    }
                }
                String sub = code.substring(j, i++);
                s.push(sub);
            } else {
                i++;
            }
        }
        return s.isEmpty();
    }
}