591. 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:

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_CONTENT. Among them, is the start tag, and 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

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 ’ ‘.

class Solution {
    public boolean isValid(String code) {
        Stack<String> stack = new Stack<>();
        for(int i = 0; i < code.length();){
            //检测 <TAG>.....</TAG>DSAJSA or 检测 DSAJSA<TAG>...</TAG>这两种情况
            if(i > 0 && stack.isEmpty()) 
                return false;
            if (code.startsWith("<![CDATA[", i)) {
                int j = i + 9;
                i = code.indexOf("]]>", j);
                if(i < 0) 
                    return false;
                i += 3;
            } else if (code.startsWith("</", i)) {
                int j = i + 2;
                i = code.indexOf('>', j);
                if(i < 0 || i == j || i - j > 9)
                    return false;
                for(int k = j; k < i; k++){
                    if(!Character.isUpperCase(code.charAt(k))) 
                        return false;
                }
                String s = code.substring(j, i++);
                if (stack.isEmpty() || !stack.pop().equals(s)) 
                    return false;
            } else if (code.startsWith("<", i)) {
                int j = i + 1;
                i = code.indexOf('>', j);
                if(i < 0 || i == j || i - j > 9) 
                    return false;
                for(int k = j; k < i; k++) {
                    if(!Character.isUpperCase(code.charAt(k))) 
                        return false;
                }
                String s = code.substring(j, i++);
                stack.push(s);
            } else {
                i++;
            }
        }
        return stack.isEmpty();
    }
}