LeetCode 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 ' '.

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本题需要写一个检查简单html源代码标签是否匹配的程序。规则有很多，8条，比如tag长度必须在[1,9]，且必须是大写字母，还有<![CDATA[CDATA_CONTENT]]>这个东西，经常会在html源代码中看见。 这题在比赛的时候没想清楚，思路比较乱。比赛结束之后，看了讨论区的解答，感觉思路好清晰，借鉴过来。 需要检查的主要有两个部分，一是Tag标签匹配，二是cdata。对于cdata，只要找到，则可以过滤掉<![CDATA[和]]>之间的所有内容。对于Tag标签匹配，类似括号匹配，可以用栈来实现。 但是因为Tag和cdata的开头都有<，我们匹配的时候， 应该从特殊到一般，即首先考虑是<![CDATA，其次考虑是</，然后考虑是<，最后就是一般性的字符了。 当遇到<![CDATA时，找到对应的]]>，下标快速跳到]]>的下一位。 当遇到</时，说明遇到了一个结束Tag，此时判断Tag是否合法，如果合法，从栈中弹出开始Tag，判断开始Tag和结束Tag是否匹配。 当遇到<时，说明遇到了一个开始Tag，判断Tag是否合法，如果合法，则压栈。 如果是一般性的字符，下标直接向前进。 需要注意的是，如果i!=0但是栈为空，说明当前的字符不在某个Tag标签内，比如这个样例：<![CDATA[wahaha]]]><![CDATA[]> wahaha]]>，一上来就是cdata，这是不合法的，因为所有内容都应该在标签对内。 完整代码如下： [cpp] class Solution { private: bool checkTag(const string &tag) { if (tag.size() < 1 || tag.size() > 9)return false; for (const auto &c : tag) { if (!isupper(c))return false; } return true; } public: bool isValid(string code) { stack<string> stk; int i = 0; while (i < code.size()) { if (i > 0 && stk.empty())return false; if (code.substr(i, 9) == "<![CDATA[") { int end = code.find("]]>", i); if (end == string::npos)return false; i = end + 3; } else if (code.substr(i, 2) == "</") { int end = code.find(‘>’, i); if (end == string::npos)return false; string tag = code.substr(i + 2, end – i – 2); if (!checkTag(tag))return false; if (stk.empty() || stk.top() != tag)return false; else stk.pop(); i = end + 1; } else if (code[i] == ‘<‘) { int end = code.find(‘>’, i); if (end == string::npos)return false; string tag = code.substr(i + 1, end – i – 1); if (!checkTag(tag))return false; stk.push(tag); i = end + 1; } else { ++i; } } return stk.empty(); } }; [/cpp] 本代码提交AC，用时12MS。]]>