题目描述：

LeetCode 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 ' '.

题目大意：

给定字符串判断是否为有效标签

整个字符串应为一个闭合标签

闭合标签的格式为<TAG_NAME>TAG_CONTENT</TAG_NAME>，其中<TAG_NAME>为标签头，</TAG_NAME>为标签尾，TAG_CONTENT为标签内容。

TAG_NAME只应当包含大写字母，长度1-9

TAG_CONTENT可包含其他有效的闭合标签，cdata以及其他有效字符，不可以包含失配的'<'、标签头和标签尾

标签头如果没有同名标签尾相互匹配，即为失配，反之亦然。然而，同时还需要考虑嵌套标签的匹配问题。

'<'如果没有'>'相互匹配，即视为失配。当遇到'<'或者'</'时，到下一个'>'之前的子串视为TAG_NAME（不一定有效）

cdata格式为：<![CDATA[CDATA_CONTENT]]>。CDATA_CONTENT为<![CDATA[与第一个]]>之间的部分。

CDATA_CONTENT可以包含任意字符。cdata的目的是防止校验器解析CDATA_CONTENT，因此即使其中包含的字符可以解析为标签，也还是将其视为普通字符。

解题思路：

栈（Stack）

可以参考括号匹配的解题思路。

Python代码：

class Solution(object):
    def isValid(self, code):
        """
        :type code: str
        :rtype: bool
        """
        tagStack = []
        while code:
            if code.startswith('<![CDATA['): #CDATA
                if not tagStack: return False
                nextIdx = code.find(']]>')
                if nextIdx == -1: return False
                code = code[nextIdx + 3:]
            elif code.startswith('</'):
                nextIdx = code.find('>')
                if nextIdx == -1: return False
                tagName = code[2: nextIdx]
                if not tagStack or tagStack.pop()!= tagName: return False
                code = code[nextIdx + 1:]
                if not tagStack: return not code
            elif code.startswith('<'):
                nextIdx = code.find('>')
                if nextIdx == -1: return False
                tagName = code[1: nextIdx]
                if not re.match('^[A-Z]{1,9}$', tagName): return False
                tagStack.append(tagName)
                code = code[nextIdx + 1:]
            elif not tagStack: return False
            else: code = code[1:]
        return not tagStack

 

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