有效的括号

 

class Solution:
    def isValid(self, s: str) -> bool:
        
        if s is None:
            return True
        
        stack = []
        flag = {
            ']': '[',
            ')': '(',
            '}': '{'
        }
        endFlag = [']', ')', '}']
        
        for i in range(len(s)):
            if s[i] not in endFlag:
                stack.append(s[i])
            else:
                if not stack:
                    return False
                elif stack.pop() != flag[s[i]]:
                    return False
        if not stack:
            return True