LeetCode in Kotlin

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:

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.

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> ”

Output: false

Explanation: Unbalanced. If “” is closed, then “” must be unmatched, and vice versa.

Constraints:

• 1 <= code.length <= 500
• code consists of English letters, digits, '<', '>', '/', '!', '[', ']', '.', and ' '.

Solution

import java.util.ArrayDeque
import java.util.Deque

class Solution {
    fun isValid(code: String): Boolean {
        val stack: Deque<String> = ArrayDeque()
        var i = 0
        while (i < code.length) {
            if (i > 0 && stack.isEmpty()) {
                return false
            }
            if (code.startsWith("<![CDATA[", i)) {
                // "<![CDATA[" length is 9
                val j = i + 9
                i = code.indexOf("]]>", j)
                if (i < 0) {
                    return false
                }
                // "]]>" length is 3
                i += 3
            } else if (code.startsWith("</", i)) {
                val j = i + 2
                i = code.indexOf(">", j)
                if (i < 0 || i == j || i - j > 9) {
                    return false
                }
                for (k in j until i) {
                    if (!Character.isUpperCase(code[k])) {
                        return false
                    }
                }
                val s = code.substring(j, i++)
                if (stack.isEmpty() || stack.pop() != s) {
                    return false
                }
            } else if (code.startsWith("<", i)) {
                val j = i + 1
                i = code.indexOf(">", j)
                if (i < 0 || i == j || i - j > 9) {
                    return false
                }
                for (k in j until i) {
                    if (!Character.isUpperCase(code[k])) {
                        return false
                    }
                }
                val s = code.substring(j, i++)
                stack.push(s)
            } else {
                i++
            }
        }
        return stack.isEmpty()
    }
}

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