LeetCode020 Valid Parentheses

Given a string 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.

Note that an empty string is also considered valid.

Example 1:

Input: "()"
Output: true

Example 2:

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

Example 3:

Input: "(]"
Output: false

Example 4:

Input: "([)]"
Output: false

Example 5:

Input: "{[]}"
Output: true

class Solution(object):
    def isValid(self, s):
        """
        :type s: str
        :rtype: bool
        """
        dic ={'(':')','{':'}','[':']'}
        
        lis=[]                       #保存s中括号
        for charactor in s:
            if charactor in dic.keys():   #如果是'(','[','{'，则保存到lis中
                lis.append(charactor)
            if charactor in dic.values():  #如果是')',']','}'
                if len(lis)==0:            #如果lis为空，也就是lis中没有保存'(','[','{'
                    return False
                elif dic[lis[-1]] == charactor: #如果lis之前保存的最后一项正好对应上这个符号
                    lis.pop()
                else:
                    return False
        
        return len(lis) == 0