How do you solve Tag Validator on LeetCode?

Tag Validator (LeetCode #591) can be solved using Brute Force or Optimal Solution. The optimal approach is Optimal Solution with O(n) time complexity and O(n) space complexity. Key insight: Using a stack is crucial for managing nested structures effectively.

What is the brute force solution for Tag Validator?

The brute force approach for Tag Validator is Brute Force, with O(n²) time complexity and O(1) space complexity. The brute-force approach involves scanning the string character by character, checking for valid tags and their contents. This method is straightforward but inefficient, as it may require multiple passes over the string to validate nested tags. The optimal Optimal Solution approach improves this to O(n).

What algorithm or data structure is used to solve Tag Validator?

Tag Validator uses the following concepts: String, Stack. The recommended patterns to study are: Stack, String Parsing.

What are the interview tips for Tag Validator?

Clearly explain your thought process and how you handle edge cases. Use a stack for problems involving nested structures; it's a common pattern. Practice writing clean and concise code, focusing on readability.

What are common mistakes when solving Tag Validator?

Failing to handle CDATA sections correctly. Not checking for unmatched tags or incorrect tag names.

#591

Tag Validator

Hard

StringStackStackString Parsing

LeetCode ↗

Approaches

Brute ForceOptimal

Complexity Comparison

	Brute Force	Optimal Solution★
Time	O(n²)	O(n)
Space	O(1)	O(n)

💡

Intuition

Time O(n)Space O(n)

The optimal solution uses a stack to efficiently manage open tags and validate nested structures in a single pass through the string. This reduces unnecessary checks and allows for quick validation of tags.

⚙️

Algorithm

6 steps

1Step 1: Initialize an empty stack and a pointer to traverse the string.
2Step 2: While traversing, check for '<' to identify tags.
3Step 3: For each start tag, validate the tag name and push it onto the stack.
4Step 4: For each end tag, check if it matches the top of the stack and pop if valid.
5Step 5: Handle CDATA sections by skipping over them without pushing to the stack.
6Step 6: After processing, ensure the stack is empty for valid tag structure.

solution.py27 lines

1def isValid(code):
2    stack = []
3    i = 0
4    while i < len(code):
5        if code[i:i+9] == '<![CDATA[':
6            i = code.find(']]>', i) + 3
7            if i == 2:
8                return False
9        elif code[i] == '<':
10            j = code.find('>', i)
11            if j == -1:
12                return False
13            tag = code[i:j+1]
14            if tag.startswith('</'):
15                tag_name = tag[2:-1]
16                if not stack or stack[-1] != tag_name:
17                    return False
18                stack.pop()
19            else:
20                tag_name = tag[1:-1]
21                if not (1 <= len(tag_name) <= 9) or not tag_name.isupper():
22                    return False
23                stack.append(tag_name)
24            i = j + 1
25        else:
26            i += 1
27    return len(stack) == 0

ℹ

Complexity note: The complexity is O(n) because we make a single pass through the string, processing each character once, and the stack can grow with the number of open tags.

1Using a stack is crucial for managing nested structures effectively.
2Validating tag names and ensuring they follow the rules is essential for correctness.

Solutions and explanations are original Tejav content. Problem titles © LeetCode — use the LeetCode button above for the full problem statement.