1. Description

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.

Note:

For simplicity, you could assume the input code (including the any characters mentioned above) only contain letters, digits, '<','>','/','!','[',']' and ' '.

2. Runtime Distribution

3. Submission Details

4. Example

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

5. Code

public boolean isValid(String code) {

    if (code == null || code.length() == 0) {
        return true;
    }

    if (code.startsWith("<!") || !code.startsWith("<")) {
        return false;
    }

    Stack<String> stack = new Stack<String>();

    String sub = "";
    for (int i = 0; i < code.length(); i++) {

        sub += code.charAt(i);
        if (!sub.startsWith("<")) {
            sub = "";
            continue;
        }

        if (sub.startsWith("<![CDATA[")) {
            if (sub.endsWith("]]>")) {
                sub = "";
            }
        } else if (sub.startsWith("</")) {
            if (sub.endsWith(">")) {
                String tagName = sub.substring(2, sub.length() - 1);
                if (stack.isEmpty() || !stack.pop().equals(tagName)) {
                    return false;
                }

                if (i < code.length() - 1 && stack.isEmpty()) {
                    return false;
                }
                sub = "";
            }
        } else if (sub.startsWith("<")) {
            if (sub.endsWith(">")) {
                String tagName = sub.substring(1, sub.length() - 1);
                if (!checkTagName(tagName)) {
                    return false;
                }
                stack.add(tagName);
                sub = "";
            }
        }
    }

    return stack.isEmpty() && sub.length() == 0;
}

private boolean checkTagName(String tagName) {
    if (tagName.length() > 9 || tagName.length() < 1) {
        return false;
    }
    for (int i = 0; i < tagName.length(); i++) {
        if (!Character.isUpperCase(tagName.charAt(i))) {
            return false;
        }
    }
    return true;
}

6.Test

import java.util.Stack;

public class LeetCode0591 {

    public boolean isValid(String code) {

        if (code == null || code.length() == 0) {
            return true;
        }

        if (code.startsWith("<!") || !code.startsWith("<")) {
            return false;
        }

        Stack<String> stack = new Stack<String>();

        String sub = "";
        for (int i = 0; i < code.length(); i++) {

            sub += code.charAt(i);
            if (!sub.startsWith("<")) {
                sub = "";
                continue;
            }

            if (sub.startsWith("<![CDATA[")) {
                if (sub.endsWith("]]>")) {
                    sub = "";
                }
            } else if (sub.startsWith("</")) {
                if (sub.endsWith(">")) {
                    String tagName = sub.substring(2, sub.length() - 1);
                    if (stack.isEmpty() || !stack.pop().equals(tagName)) {
                        return false;
                    }

                    if (i < code.length() - 1 && stack.isEmpty()) {
                        return false;
                    }
                    sub = "";
                }
            } else if (sub.startsWith("<")) {
                if (sub.endsWith(">")) {
                    String tagName = sub.substring(1, sub.length() - 1);
                    if (!checkTagName(tagName)) {
                        return false;
                    }
                    stack.add(tagName);
                    sub = "";
                }
            }
        }

        return stack.isEmpty() && sub.length() == 0;
    }

    private boolean checkTagName(String tagName) {
        if (tagName.length() > 9 || tagName.length() < 1) {
            return false;
        }
        for (int i = 0; i < tagName.length(); i++) {
            if (!Character.isUpperCase(tagName.charAt(i))) {
                return false;
            }
        }
        return true;
    }

    public static void main(String[] args) {
        LeetCode0591 leetcode = new LeetCode0591();
        System.out.println(leetcode.isValid("<A></A><B></B>"));
    }
}