591 - Tag Validator.

Hard

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:

• 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.

Example 1:

Input: code = "<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 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 = "<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.

Example 3:

Input: code = "<A><B></A></B>"

Output: false

Explanation: Unbalanced. If "<A>" is closed, then "<B>" must be unmatched, and vice versa.

Constraints:

• 1 <= code.length <= 500

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