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Problem solution · Python

Tag Validator

Tag Validator: a Python solution using direct simulation. Learn the idea, check the complexity, and read the full code, with credit to walkccc LeetCode Solutions.

Technique
Direct simulation
Source
walkccc LeetCode Solutions
Length
52 lines
Start with the idea.

Try the problem first. If you get stuck, read the approach below, then write your own solution. The full code is at the bottom.

Approach

Direct simulation

For Tag Validator, the implementation follows the problem’s operations directly while maintaining only the state needed for the next decision.

  1. Translate each rule into one explicit state update.
  2. Maintain the invariant after every processed item.
  3. Return the accumulated state once all relevant input has been handled.

Code notes

  • 52 lines of Python from the credited upstream file 591.py.
  • The implementation visibly relies on sequence storage.
  • No explicit loop blocks detected.

Complexity

Count the number and nesting of passes over the input, then include the maintained containers in the memory estimate.

Check the problem constraints before deciding whether this complexity will pass.

Source

Code and credit

This code comes from walkccc LeetCode Solutions by P.-Y. Chen (walkccc) and is used under the MIT licence.

Open problem ↗Original source ↗
Full codeTag Validator · PythonPython
Use this to learn the idea, then write your own version.
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] == '<':        # It's inside a tag, so check if it's a cdata.        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] == '/':  # the end tag          closeIndex = code.find('>', i + 2)          if closeIndex == -1 or not isValidTagName(                  code[i + 2: closeIndex],                  True):            return False        else:  # the start tag          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

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