[LeetCode with Python] 20. Valid Parenthesis

First Approach: use 'stack' and 'dictionary' to check correct parenthesis pair

 

from collections import deque

par_map = {')': '(', '}': '{', ']': '['}

class Solution:
    def isValid(self, s: str) -> bool:
        stack = deque()
        
        for letter in s:
            if letter not in par_map:
                stack.append(letter)
            else:
                if not stack:
                    return False
                elif stack[-1] == par_map[letter]:
                    stack.pop()
                else:
                    return False
        if stack:
            return False
        return True

 

Solution: use stack too, but also pythonic skills for efficiency

class Solution(object):
    def isValid(self, s):
        """
        :type s: str
        :rtype: bool
        """

        # The stack to keep track of opening brackets.
        stack = []

        # Hash map for keeping track of mappings. This keeps the code very clean.
        # Also makes adding more types of parenthesis easier
        mapping = {")": "(", "}": "{", "]": "["}

        # For every bracket in the expression.
        for char in s:

            # If the character is an closing bracket
            if char in mapping:

                # Pop the topmost element from the stack, if it is non empty
                # Otherwise assign a dummy value of '#' to the top_element variable
                top_element = stack.pop() if stack else '#'

                # The mapping for the opening bracket in our hash and the top
                # element of the stack don't match, return False
                if mapping[char] != top_element:
                    return False
            else:
                # We have an opening bracket, simply push it onto the stack.
                stack.append(char)

        # In the end, if the stack is empty, then we have a valid expression.
        # The stack won't be empty for cases like ((()
        return not stack