Merge pull request #1374 from shreyash3087/add/leetcode-591

Docs: Added Solutions to Leetcode 591

Author: ajay-dhangar

dsa-problems/leetcode-problems/0500-0599.md

@@ -476,7 +476,7 @@ export const problems =[
     "problemName": "591. Tag Validator",
     "difficulty": "Hard",
     "leetCodeLink": "https://leetcode.com/problems/tag-validator",
-    "solutionLink": "#"
+    "solutionLink": "/dsa-solutions/lc-solutions/0500-0599/tag-validator"
   },
   {
     "problemName": "592. Fraction Addition and Subtraction",

dsa-solutions/lc-solutions/0500-0599/0591-tag-validator.md

@@ -0,0 +1,292 @@
+---
+id: tag-validator
+title: Tag Validator
+sidebar_label: 0591 - Tag Validator
+tags:
+  - Iterator
+  - Stack
+  - String
+description: "This is a solution to the Tag Validator problem on LeetCode."
+---
+
+## Problem Description
+
+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**.
+
+### Examples
+**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.
+```
+
+### Constraints
+
+- `1 <= code.length <= 500`
+- `code` consists of English letters, digits, `'<'`, `'>'`, `'/'`, `'!'`, `'['`, `']'`, `'.'`, and `' '`.
+
+## Solution for Tag Validator
+
+### Approach: Stack
+Summarizing the given problem, we can say that we need to determine whether a tag is valid or not, by checking the following properties.
+
+1. The code should be wrapped in a valid closed tag.
+
+2. The `TAG_NAME` should be valid.
+
+3. The `TAG_CONTENT` should be valid.
+
+4. The cdata should be valid.
+
+5. All the tags should be closed. i.e. each start-tag should have a corresponding end-tag and vice-versa and the order of the tags should be correct as well.
+
+In order to check the validity of all these, firstly, we need to identify which parts of the given code string act as which part from the above-mentioned categories. To understand how it's done, we'll go through the implementation and the reasoning behind it step by step.
+
+We iterate over the given code string. Whenever a `<` is encountered(unless we are currently inside `<![CDATA[...]]>`), it indicates the beginning of either a `TAG_NAME`(start tag or end tag) or the beginning of cdata as per the conditions given in the problem statement.
+
+If the character immediately following this `<` is an `!`, the characters following this `<` can't be a part of a valid `TAG_NAME`, since only upper-case letters(in case of a start tag) or `/` followed by upper-case letters(in the case of an end tag). Thus, the choice now narrows down to only **cdata**. Thus, we need to check if the current bunch of characters following `<!`(including it) constitute a valid **cdata**. To do this, firstly we find out the first matching `]]>` following the current `<!` to mark the ending of **cdata**. If no such matching `]]>` exists, the code string is considered as invalid. Apart from this, the `<!` should also be immediately followed by `CDATA[` for the **cdata** to be valid. The characters lying inside the `<![CDATA[` and `]]>` do not have any constraints on them.
+
+If the character immediately following the `<` encountered isn't an `!`, this `<` can only mark the beginning of `TAG_NAME`. Now, since a valid start tag can't contain anything except upper-case letters if a `/` is found after `<`, the `</` pair indicates the beginning of an end tag. Now, when a `<` refers to the beginning of a `TAG_NAME`(either start-tag or end-tag), we find out the first closing `>` following the `<` to find out the substring(say s), that constitutes the `TAG_NAME`. This s should satisfy all the criteria to constitute a valid `TAG_NAME`. Thus, for every such s, we check if it contains all upper-case letters and also check its length(It should be between 1 to 9). If any of the criteria isn't fulfilled, s doesn't constitute a valid `TAG_NAME`. Hence, the code string turns out to be invalid as well.
+
+Apart from checking the validity of the `TAG_NAME`, we also need to ensure that the tags always exist in pairs. i.e. for every start-tag, a corresponding end-tag should always exist. Further, we can note that in case of multiple `TAG_NAME`'s, the `TAG_NAME` whose start-tag comes later than the other ones, should have its end-tag appearing before the end-tags of those other `TAG_NAME`'s. i.e. the tag that starts later should end first.
+
+From this, we get the intuition that we can make use of a stack to check the existence of matching start and end-tags. Thus, whenever we find out a valid start-tag, as mentioned above, we push its `TAG_NAME` string onto a stack. Now, whenever an end-tag is found, we compare its `TAG_NAME` with the `TAG_NAME` at the top of the stack and remove this element from the stack. If the two don't match, this implies that either the current end-tag has no corresponding start-tag or there is a problem with the ordering of the tags. The two need to match for the tag-pair to be valid since there can't exist an end-tag without a corresponding start-tag and vice-versa. Thus, if a match isn't found, we can conclude that the given code string is invalid.
+
+Now, after the complete code string has been traversed, the stack should be empty if all the start-tags have their corresponding end-tags as well. If the stack isn't empty, this implies that some start-tag doesn't have the corresponding end-tag, violating the closed-tag's validity condition.
+
+Further, we also need to ensure that the given code is completely enclosed within closed tags. For this, we need to ensure that the first **cdata** found is also inside the closed tags. Thus, when we find a possibility of the presence of **cdata**, we proceed further only if we've already found a start tag, indicated by a non-empty stack. Further, to ensure that no data lies after the last end-tag, we need to ensure that the stack doesn't become empty before we reach the end of the given code string since an empty stack indicates that the last end-tag has been encountered.
+## Code in Different Languages
+
+<Tabs>
+<TabItem value="cpp" label="C++">
+  <SolutionAuthor name="@Shreyash3087"/>
+
+```cpp
+#include <stack>
+#include <string>
+
+class Solution {
+private:
+    std::stack<std::string> stack;
+    bool contains_tag = false;
+
+    bool isValidTagName(std::string s, bool ending) {
+        if (s.length() < 1 || s.length() > 9)
+            return false;
+        for (int i = 0; i < s.length(); i++) {
+            if (!isupper(s[i]))
+                return false;
+        }
+        if (ending) {
+            if (!stack.empty() && stack.top() == s)
+                stack.pop();
+            else
+                return false;
+        } else {
+            contains_tag = true;
+            stack.push(s);
+        }
+        return true;
+    }
+
+    bool isValidCdata(std::string s) {
+        return s.find("[CDATA[") == 0;
+    }
+
+public:
+    bool isValid(std::string code) {
+        if (code[0] != '<' || code[code.length() - 1] != '>')
+            return false;
+        for (int i = 0; i < code.length(); i++) {
+            bool ending = false;
+            int closeindex;
+            if (stack.empty() && contains_tag)
+                return false;
+            if (code[i] == '<') {
+                if (!stack.empty() && code[i + 1] == '!') {
+                    closeindex = code.find("]]>", i + 1);
+                    if (closeindex < 0 || !isValidCdata(code.substr(i + 2, closeindex - i - 2)))
+                        return false;
+                } else {
+                    if (code[i + 1] == '/') {
+                        i++;
+                        ending = true;
+                    }
+                    closeindex = code.find('>', i + 1);
+                    if (closeindex < 0 || !isValidTagName(code.substr(i + 1, closeindex - i - 1), ending))
+                        return false;
+                }
+                i = closeindex;
+            }
+        }
+        return stack.empty() && contains_tag;
+    }
+};
+
+
+```
+</TabItem>
+<TabItem value="java" label="Java">
+  <SolutionAuthor name="@Shreyash3087"/>
+
+```java
+public class Solution {
+    Stack < String > stack = new Stack < > ();
+    boolean contains_tag = false;
+    public boolean isValidTagName(String s, boolean ending) {
+        if (s.length() < 1 || s.length() > 9)
+            return false;
+        for (int i = 0; i < s.length(); i++) {
+            if (!Character.isUpperCase(s.charAt(i)))
+                return false;
+        }
+        if (ending) {
+            if (!stack.isEmpty() && stack.peek().equals(s))
+                stack.pop();
+            else
+                return false;
+        } else {
+            contains_tag = true;
+            stack.push(s);
+        }
+        return true;
+    }
+    public boolean isValidCdata(String s) {
+        return s.indexOf("[CDATA[") == 0;
+    }
+    public boolean isValid(String code) {
+        if (code.charAt(0) != '<' || code.charAt(code.length() - 1) != '>')
+            return false;
+        for (int i = 0; i < code.length(); i++) {
+            boolean ending = false;
+            int closeindex;
+            if(stack.isEmpty() && contains_tag)
+                return false;
+            if (code.charAt(i) == '<') {
+                if (!stack.isEmpty() && code.charAt(i + 1) == '!') {
+                    closeindex = code.indexOf("]]>", i + 1);
+                    if (closeindex < 0 || !isValidCdata(code.substring(i + 2, closeindex)))
+                        return false;
+                } else {
+                    if (code.charAt(i + 1) == '/') {
+                        i++;
+                        ending = true;
+                    }
+                    closeindex = code.indexOf('>', i + 1);
+                    if (closeindex < 0 || !isValidTagName(code.substring(i + 1, closeindex), ending))
+                        return false;
+                }
+                i = closeindex;
+            }
+        }
+        return stack.isEmpty() && contains_tag;
+    }
+}
+```
+
+</TabItem>
+<TabItem value="python" label="Python">
+  <SolutionAuthor name="@Shreyash3087"/>
+
+```python
+class Solution:
+    def __init__(self):
+        self.stack = []
+        self.contains_tag = False
+
+    def isValidTagName(self, s, ending):
+        if len(s) < 1 or len(s) > 9:
+            return False
+        if not all(c.isupper() for c in s):
+            return False
+        if ending:
+            if self.stack and self.stack[-1] == s:
+                self.stack.pop()
+            else:
+                return False
+        else:
+            self.contains_tag = True
+            self.stack.append(s)
+        return True
+
+    def isValidCdata(self, s):
+        return s.startswith("[CDATA[")
+
+    def isValid(self, code):
+        if code[0] != '<' or code[-1] != '>':
+            return False
+        i = 0
+        while i < len(code):
+            ending = False
+            if not self.stack and self.contains_tag:
+                return False
+            if code[i] == '<':
+                if self.stack and code[i + 1] == '!':
+                    closeindex = code.find("]]>", i + 1)
+                    if closeindex < 0 or not self.isValidCdata(code[i + 2:closeindex]):
+                        return False
+                else:
+                    if code[i + 1] == '/':
+                        i += 1
+                        ending = True
+                    closeindex = code.find('>', i + 1)
+                    if closeindex < 0 or not self.isValidTagName(code[i + 1:closeindex], ending):
+                        return False
+                i = closeindex
+            i += 1
+        return not self.stack and self.contains_tag
+
+
+```
+</TabItem>
+</Tabs>
+
+## Complexity Analysis
+
+### Time Complexity: $O(N)$
+
+> **Reason**: We traverse over the given code string of length N.
+
+### Space Complexity: $O(N)$
+
+> **Reason**: The stack can grow upto a size of n/3 in the worst case. e.g. In case of `<A><B><C><D>`, N=12 and number of tags = 12/3 = 4.
+
+## References
+
+- **LeetCode Problem**: [Tag Validator](https://leetcode.com/problems/tag-validator/description/)
+
+- **Solution Link**: [Tag Validator](https://leetcode.com/problems/tag-validator/solutions/)