README.md

591. Tag Validator

Given a string representing a code snippet, 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.

Example 1:

Input: code = "
This is the first line ]]>
"
Output: true
Explanation:
The code is wrapped in a closed tag : 
 and 
.
The TAG_NAME is valid, the TAG_CONTENT consists of some characters and cdata.
Although CDATA_CONTENT has an unmatched start tag with invalid TAG_NAME, it should be considered as plain text, not parsed as a tag.
So TAG_CONTENT is valid, and then the code is valid. Thus return true.

Example 2:

Input: code = "
>>  ![cdata[]] ]>]]>]]>>]
"
Output: true
Explanation:
We first separate the code into : start_tag|tag_content|end_tag.
start_tag -> "
"
end_tag -> "
"
tag_content could also be separated into : text1|cdata|text2.
text1 -> ">>  ![cdata[]] "
cdata -> "]>]]>", where the CDATA_CONTENT is "
]>"
text2 -> "]]>>]"
The reason why start_tag is NOT "
>>" is because of the rule 6.
The reason why cdata is NOT "]>]]>]]>" is because of the rule 7.

Example 3:

Input: code = "      "
Output: false
Explanation: Unbalanced. If "" is closed, then "" must be unmatched, and vice versa.

Constraints:

• 1 <= code.length <= 500
• code consists of English letters, digits, '<', '>', '/', '!', '[', ']', '.', and ' '.

Solutions (Python)

1. Solution

class Solution:
    def isValid(self, code: str) -> bool:
        cdata = False
        tagstack = []
        i = 0

        while i < len(code):
            if cdata:
                if code[i:i + 3] == "]]>":
                    cdata = False
                    i += 2
            elif tagstack == [] and (code[i] != '<' or code[i:i + 2] in "</<!"):
                return False
            elif code[i:i + 9] == "<![CDATA[":
                cdata = True
                i += 8
            elif code[i:i + 2] == "</":
                for j in range(i + 2, i + 13):
                    if j >= len(code) or j == i + 12 or (j == i + 2 and code[j] == '>'):
                        return False
                    elif code[j] == '>':
                        if tagstack.pop() != code[i + 2:j]:
                            return False
                        if tagstack == [] and j != len(code) - 1:
                            return False
                        i = j
                        break
                    elif not code[j].isupper():
                        return False
            elif code[i] == '<':
                for j in range(i + 1, i + 12):
                    if j >= len(code) or j == i + 11 or (j == i + 1 and code[j] == '>'):
                        return False
                    elif code[j] == '>':
                        tagstack.append(code[i + 1:j])
                        i = j
                        break
                    elif not code[j].isupper():
                        return False

            i += 1

        return tagstack == []