20. Valid Parentheses - leetcode - python - easy

 

Given a string s containing just the characters '(', ')', '{', '}', '[' and ']', determine if the input string is valid.

An input string is valid if:

1. Open brackets must be closed by the same type of brackets.
2. Open brackets must be closed in the correct order.
3. Every close bracket has a corresponding open bracket of the same type.

 

Example 1:

Input: s = "()"

Output: true

Example 2:

Input: s = "()[]{}"

Output: true

Example 3:

Input: s = "(]"

Output: false

Example 4:

Input: s = "([])"

Output: true

Example 5:

Input: s = "([)]"

Output: false

 

Constraints:

• 1 <= s.length <= 104
• s consists of parentheses only '()[]{}'.

class Solution:

    def isValid(self, s: str) -> bool:

        stack = []

        mapping = {")":"(", "}":"{", "]":"["}

        for c in s:

            if c in mapping.values():

                stack.append(c)

            elif not stack or stack.pop() != mapping[c]:

                return False

        return not stack

Explanation (Simple Version) Use a stack to keep opening brackets. For each character: If it’s an opening bracket → push to stack If it’s a closing bracket → check if it matches the top of stack If any mismatch → return False. At the end, stack should be empty → return True.