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.
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

 

Constraints:

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

 

code by python:

class Solution:
    def isValid(self, s: str) -> bool:
        stack = []
        # not to be confused, use the dictionary
        dic = {
            ')': '(',
            '}': '{',
            ']': '['
        }

        #the contents to append to the stack
        opener = "({["

        for c in s:
            if c in opener:
                stack.append(c)
            else:
                # stack case like [')', ] or ['}', ] ...
                if not stack:
                    return False
                top = stack.pop()

                # stack case like ['(', ']'] or ['{', ')'] ...
                if dic[c] != top:
                    return False
        # all of 's' was scanned
        # If there are elements left on the stack, have to return False. else True
        # stack case like ['(', '['] or ['{' ...] ...
        return not stack