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:

The code must be wrapped in a valid closed tag. Otherwise, the code is invalid.
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.
A valid TAG_NAME only contain upper-case letters, and has length in range [1,9]. Otherwise, the TAG_NAME is invalid.
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.
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.
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).
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 ]]>. 
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:

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

Source: leetcode | Difficulty: MEDIUM_HARD
Tags: String algorithms

Data structures

Starter code: class Solution:
    def isValid(self, code: str) -> bool:
        

Solutions:
class Solution:

	def isValid(self, code):

		def parseTag(src, i):
			j = i
			(tag, i) = findtag(src, i)
			if not tag:
				return (False, j)
			(res, i) = parseContent(src, i)
			e = i + len(tag) + 3
			return (True, e) if src[i:e] == '</' + tag + '>' else (False, j)

		def parseContent(src, i):
			(j, res) = (i, False)
			while i < len(src):
				(res, i) = parseText(src, i)
				if res:
					continue
				(res, i) = parseCDData(src, i)
				if res:
					continue
				(res, i) = parseTag(src, i)
				if res:
					continue
				break
			return (True, i)

		def parseCDData(src, i):
			s = src.find('<![CDATA[', i)
			if s != i:
				return (False, i)
			e = src.find(']]>', i)
			return (True, e + 3) if e != -1 else (False, i)

		def parseText(src, i):
			j = i
			while i < len(src) and src[i] != '<':
				i += 1
			return (j != i, i)

		def findtag(src, i):
			if src[i] != '<':
				return ('', i)
			e = src.find('>', i + 1)
			if e == -1 or e - i - 1 > 9 or e - i - 1 < 1:
				return ('', e)
			s = 1
			while s < e - i and src[i + s].isupper():
				s += 1
			return (src[i + 1:e], e + 1) if s >= e - i else ('', e)
		return parseTag(code, 0) == (True, len(code))


class Solution:

	def isValid(self, code):
		S = code
		CDATA_BEGIN = '![CDATA['
		CDATA_END = ']]>'

		def collect_tag(i):
			for j in range(i, len(S)):
				if S[j] == '>':
					break
			else:
				return None
			return S[i + 1:j]

		def valid_tag(tag):
			if 1 <= len(tag) <= 9 and all(('A' <= c <= 'Z' for c in tag)):
				return True
			else:
				return False
		if not S or S[0] != '<':
			return False
		tag = collect_tag(0)
		if not tag or not S.startswith('<{}>'.format(tag)) or (not S.endswith('</{}>'.format(tag))) or (not valid_tag(tag)):
			return False
		S = S[len(tag) + 2:-len(tag) - 3]
		i = 0
		stack = []
		while i < len(S):
			if S[i] == '<':
				tag = collect_tag(i)
				if not tag:
					return False
				if tag.startswith(CDATA_BEGIN):
					while i < len(S) and S[i:i + 3] != CDATA_END:
						i += 1
					if S[i:i + 3] != CDATA_END:
						return False
					i += 2
				elif tag.startswith('/'):
					tag = tag[1:]
					if not valid_tag(tag) or not stack or stack.pop() != tag:
						return False
				else:
					if not valid_tag(tag):
						return False
					stack.append(tag)
			i += 1
		return not stack