20-Valid-Parentheses.md

20. Valid Parentheses

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.

Example 1:

Input: s = "()"
Output: true

Example 2:

Input: s = "()[]{}"
Output: true

Example 3:

Input: s = "(]"
Output: false

Example 4:

Input: s = "([)]"
Output: false

Example 5:

Input: s = "{[]}"
Output: true

Constraints:

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

Solution

class Solution:
    def isValid(self, s: str) -> bool:
        stack = []
        for c in s:
            if   c == '(': stack.append(0)
            elif c == '[': stack.append(1)
            elif c == '{': stack.append(2)
            elif c == ')': 
                if not stack or stack[-1] != 0:
                    return False
                else:
                    stack.pop()
            elif c == ']': 
                if not stack or stack[-1] != 1:
                    return False
                else:
                    stack.pop()
            elif c == '}': 
                if not stack or stack[-1] != 2:
                    return False
                else:
                    stack.pop()
        return stack == []

Simple stack.