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 ' '.

Difficulty: interview

Starter code:

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

Solutions:
class Solution:
     def isValid(self, code):
         """
         :type code: str
         :rtype: bool
         """
         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)
 
         # start to check
         return parseTag(code, 0) == (True, len(code))

class Solution:
     def isValid(self, code):
         """
         :type code: str
         :rtype: bool
         """
         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
         #print('###########1111############')
         tag=collect_tag(0)
         #print(tag)
         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)
                 #print(tag)
                 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