20.valid-parentheses

class Node:
    def __init__(self, data):
        self.data = data
        self.next = None

class Stack:
    def __init__(self):
        self.top = None

    def push(self, data):
        if self.top is None:
            self.top = Node(data)
        else:
            node = Node(data)
            node.next = self.top
            self.top = node

    def pop(self):
        if self.top is None:
            return None
        node = self.top
        self.top = self.top.next
        return node.data

    def peek(self):
        if self.top is None:
            return None
        return self.top.data

    def is_empty(self):
        return self.top is None

class Solution:
    def isValid(self, s: str) -> bool:
        pair = {
            ')': '(',
            '}': '{',
            ']':'['
        }
        mystack = Stack()
        for c in s:
            if mystack.is_empty() or c not in pair.keys() or pair[c] != mystack.peek():
                mystack.push(c)
            elif pair[c] == mystack.peek():
                mystack.pop()
        return mystack.is_empty()