LeetCode Tag Validator

LeetCode 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 ' '.

本题需要写一个检查简单html源代码标签是否匹配的程序。规则有很多,8条,比如tag长度必须在[1,9],且必须是大写字母,还有<![CDATA[CDATA_CONTENT]]>这个东西,经常会在html源代码中看见。

这题在比赛的时候没想清楚,思路比较乱。比赛结束之后,看了讨论区的解答,感觉思路好清晰,借鉴过来。

需要检查的主要有两个部分,一是Tag标签匹配,二是cdata。对于cdata,只要找到,则可以过滤掉<![CDATA[和]]>之间的所有内容。对于Tag标签匹配,类似括号匹配,可以用栈来实现。

但是因为Tag和cdata的开头都有<,我们匹配的时候, 应该从特殊到一般,即首先考虑是<![CDATA,其次考虑是</,然后考虑是<,最后就是一般性的字符了。

当遇到<![CDATA时,找到对应的]]>,下标快速跳到]]>的下一位。

当遇到</时,说明遇到了一个结束Tag,此时判断Tag是否合法,如果合法,从栈中弹出开始Tag,判断开始Tag和结束Tag是否匹配。

当遇到<时,说明遇到了一个开始Tag,判断Tag是否合法,如果合法,则压栈。

如果是一般性的字符,下标直接向前进。

需要注意的是,如果i!=0但是栈为空,说明当前的字符不在某个Tag标签内,比如这个样例:<![CDATA[wahaha]]]><![CDATA[]> wahaha]]>,一上来就是cdata,这是不合法的,因为所有内容都应该在标签对内。

完整代码如下:

class Solution {
private:
	bool checkTag(const string &tag) {
		if (tag.size() < 1 || tag.size() > 9)return false;
		for (const auto &c : tag) {
			if (!isupper(c))return false;
		}
		return true;
	}
public:
	bool isValid(string code) {
		stack<string> stk;
		int i = 0;
		while (i < code.size()) {
			if (i > 0 && stk.empty())return false;
			if (code.substr(i, 9) == "<![CDATA[") {
				int end = code.find("]]>", i);
				if (end == string::npos)return false;
				i = end + 3;
			}
			else if (code.substr(i, 2) == "</") {
				int end = code.find('>', i);
				if (end == string::npos)return false;
				string tag = code.substr(i + 2, end - i - 2);
				if (!checkTag(tag))return false;
				if (stk.empty() || stk.top() != tag)return false;
				else stk.pop();
				i = end + 1;
			}
			else if (code[i] == '<') {
				int end = code.find('>', i);
				if (end == string::npos)return false;
				string tag = code.substr(i + 1, end - i - 1);
				if (!checkTag(tag))return false;
				stk.push(tag);
				i = end + 1;
			}
			else {
				++i;
			}
		}
		return stk.empty();
	}
};

本代码提交AC,用时12MS。

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