Tag Validator

1
2
3
4
5
6
7
8
class Solution:
    def isValid(self, code):
        code = re.sub(r'<!\[CDATA\[.*?\]\]>|t', '-', code)
        prev = None
        while code != prev:
            prev = code
            code = re.sub(r'<([A-Z]{1,9})>[^<]*</\1>', 't', code)
        return code == 't'

Identifying Problem Isomorphism

“Tag Validator” can be approximately mapped to “Valid Parentheses”.

Here’s why:

Both problems involve validating a string based on a set of rules. In “Tag Validator”, the rules are related to how tags are opened and closed, and the correctness of the tag’s content. In “Valid Parentheses”, the rules are related to how different types of parentheses must be correctly opened and closed.

The difference is in the complexity of the rules: “Tag Validator” has a set of complex rules, including tag opening/closing and content rules, while “Valid Parentheses” has a simple rule about correctly matching parentheses.

“Tag Validator” is more complex due to the additional rules and requirements. However, the basic concept of matching pairs (parentheses in one case, tags in the other) is a common theme between the two problems.

This is an approximate mapping because the intricacies of HTML tag validation in the “Tag Validator” problem do not have a direct equivalent in the “Valid Parentheses” problem. Despite this, the underlying concept of stack-based validation of nested structures serves as a common ground between these problems.

10 Prerequisite LeetCode Problems

“591. Tag Validator” requires knowledge of string parsing and stack data structures. Here are 10 problems to prepare for it:

These problems help to understand the use of stack for parsing and validating strings, which is the main concept required to solve “591. Tag Validator”.

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
class Solution:
  def isValid(self, code: str) -> bool:
    if code[0] != '<' or code[-1] != '>':
      return False
    containsTag = False  
    stack = []  

    def isValidCdata(s: str) -> bool:  
        return s.find('[CDATA[') == 0

    def isValidTagName(tagName: str, isEndTag: bool) -> bool:  
        nonlocal containsTag
        if not tagName or len(tagName) > 9:  
            return False
        if any(not c.isupper() for c in tagName):  
            return False

        if isEndTag:  
            return stack and stack.pop() == tagName

        containsTag = True  
        stack.append(tagName)  
        return True

    i = 0
    while i < len(code):
        if not stack and containsTag:  
            return False
        if code[i] == '<':
            if stack and code[i + 1] == '!':  
                closeIndex = code.find(']]>', i + 2)
                if closeIndex == -1 or not isValidCdata(code[i + 2:closeIndex]):  
                    return False
            elif code[i + 1] == '/':  
                closeIndex = code.find('>', i + 2)
                if closeIndex == -1 or not isValidTagName(code[i + 2:closeIndex], True):  
                    return False
            else:  
                closeIndex = code.find('>', i + 1)
                if closeIndex == -1 or not isValidTagName(code[i + 1:closeIndex], False):  
                    return False
            i = closeIndex  
        i += 1

    return not stack and containsTag  

Problem Classification

The given problem statement falls under the domain of String Parsing and Syntax Validation. In this problem, we are provided with a string that represents a code snippet, and our task is to validate whether the code follows a specific set of syntax rules or not. This involves a depth understanding of string manipulation, syntax rules, and structured data format.

The ‘What’ components of the problem are:

Input: A string representing a code snippet.
Output: A boolean value indicating whether the code snippet is valid or not based on a set of syntax rules.

Based on these, we can further classify the problem into the following categories:

String Manipulation: The problem requires us to parse and manipulate strings. We need to understand how to iterate over the characters in the string and analyze the segments based on the specific syntax rules.
Stack: The problem description hints towards a stack-based solution. The rules mention about tags being properly nested and balanced which is a common use-case for stack data structure.
Parsing: Parsing is a significant aspect of this problem. The validator needs to parse the code snippet, identify different components (tags, cdata, etc.), and ensure they follow the specified rules.
Syntax Validation: After parsing the string, the validator has to apply a set of rules to check if the syntax is valid or not. It must understand the rules regarding valid tag names, cdata content, and how to handle unmatched or incorrectly nested tags.

The complexity of this problem is quite high, given the numerous rules and edge cases that need to be handled for proper validation. Moreover, the task requires understanding of string parsing and manipulation techniques, the usage of stack for checking properly nested tags, and knowledge of syntax validation rules.

Language Agnostic Coding Drills

The code validates XML-like tags within a string. It covers multiple concepts of programming, such as loops, conditionals, string manipulation, stacks, and the usage of helper functions. Let’s break down these into smaller units of learning:

Understanding XML-like Tags: Before you dive into the problem, you need to understand how XML-like tags work. Specifically, how opening and closing tags are formed and what rules they must adhere to (for example, tag names are case-sensitive, must start with a letter, etc.).
String Manipulation: One needs to be comfortable with basic string operations like indexing, finding substrings, slicing strings, etc. In this code, various string manipulations are used to extract tag names, check for certain conditions, etc.
Conditional Statements: If-else conditions are used extensively in the code to check various conditions like if the string starts and ends with ‘<’ and ‘>’, if the stack is empty, etc.
Loops: Understanding how loops work is fundamental to this problem as we iterate over the given string to extract and validate tags.
Stack Data Structure: This problem heavily relies on the stack data structure for keeping track of the open tags and making sure they are properly closed.
Helper Functions: In this problem, helper functions are used to check if a string represents a valid CDATA or tag name. Understanding how to write and use helper functions is crucial here.
Boolean Flags: Boolean flags like containsTag are used to keep track of certain conditions throughout the code. In this case, containsTag is used to check if the code contains any tags.

To solve this problem, one can follow these steps:

First, check if the string starts and ends with ‘<’ and ‘>’. If not, return False.
Then, iterate through the string and check every character.
- If you find a ‘<’, check the following character.
  - If it’s ‘!’, it means it could be a CDATA section. Find the closing ‘]]>’ and validate the CDATA section.
  - If it’s ‘/’, it’s a closing tag. Find the ‘>’ character to get the tag name and check if it’s a valid closing tag.
  - Otherwise, it’s an opening tag. Find the ‘>’ character to get the tag name and check if it’s a valid opening tag.
At the end, if the stack is not empty (meaning there are unclosed tags) or there were no tags in the string, return False. Otherwise, return True.

It’s important to understand that these steps are interrelated and all of them must be properly understood and implemented for the solution to work correctly.

Targeted Drills in Python

Understanding XML-like Tags: For this, we’ll just write some example tags. They’re simple to understand, so there’s not really a drill for this.
String Manipulation:

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
# String Indexing
s = "Hello, World!"
print(s[7])  # Output: W

# Find Substring
index = s.find("World")
print(index)  # Output: 7

# String Slicing
substring = s[7:12]
print(substring)  # Output: World

Conditional Statements:

1
2
3
4
5
6
# If-Else Condition
x = 10
if x > 5:
    print("x is greater than 5")
else:
    print("x is not greater than 5")

Loops:

1
2
3
# For Loop
for i in range(5):
    print(i)  # Output: 0, 1, 2, 3, 4

Stack Data Structure:

1
2
3
4
5
6
# Stack operations
stack = []
stack.append('a')  # Push 'a'
stack.append('b')  # Push 'b'
print(stack.pop())  # Pop and print: 'b'
print(stack)  # Remaining stack: ['a']

Helper Functions:

1
2
3
4
5
6
7
# Define a helper function
def is_even(n):
    return n % 2 == 0

# Use the helper function
print(is_even(4))  # Output: True
print(is_even(5))  # Output: False

Boolean Flags:

1
2
3
4
# Using Boolean Flags
flag = False
if not flag:
    print("Flag is not set!")  # Output: Flag is not set!

Problem Specific Drills:

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16
# Validating CDATA section
def isValidCdata(s: str) -> bool:
    return s.find('[CDATA[') == 0

print(isValidCdata('[CDATA[ This is valid CDATA section ]]'))  # Output: True

# Validating tag names
def isValidTagName(tagName: str, isEndTag: bool) -> bool:
    if not tagName or len(tagName) > 9:  
        return False
    if any(not c.isupper() for c in tagName):  
        return False
    return True

print(isValidTagName('VALIDTAG', False))  # Output: True
print(isValidTagName('InvalidTag', False))  # Output: False

These drills cover each concept needed for the main problem. To solve the main problem, you would need to integrate these concepts and drills together.