LeetCode 591 - Tag Validator

https://leetcode.com/problems/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 ' '.

X. Sequentially build current structure, use stack for tag matching
Suggest to add this test case: <![CDATA[<div>]]>
https://discuss.leetcode.com/topic/91300/java-solution-use-startswith-and-indexof
An patterned way of parsing and validating

    public boolean isValid(String code) {
        Stack<String> stack = new Stack<>();
        for(int i = 0; i < code.length();){
            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++;// normal data
            }
        }
        return stack.isEmpty();
    }

http://blog.csdn.net/u014688145/article/details/72859739
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(); }
X. TODO
https://discuss.leetcode.com/topic/91373/java-solution-sequentially-build-current-structure-use-stack-for-tag-matching

X. Use regulare expression
https://discuss.leetcode.com/topic/91381/short-python-accepted-but-not-sure-if-correct/4
https://discuss.leetcode.com/topic/91406/java-solution-7-lines-regular-expression

Only change is in Java we need to use double \ to quote a special character.

Two brilliant points:

1. Use Non Greedy mode (.*?) when matching CDATA. Reference: https://stackoverflow.com/questions/3075130/what-is-the-difference-between-and-regular-expressions
2. Use Group (([A-Z]{1,9}) then \\1) when matching TAG_NAME. Reference: http://www.regular-expressions.info/refcapture.html

    public boolean isValid(String code) {
        if (code.equals("t")) return false;
        code = code.replaceAll("<!\\[CDATA\\[.*?\\]\\]>", "c");

        String prev = "";
        while (!code.equals(prev)) {
            prev = code;
            code = code.replaceAll("<([A-Z]{1,9})>[^<]*</\\1>", "t");
        }

        return code.equals("t");
    }