有效括号-正确的顺序闭合

'''
有效括号:
左括号必须以正确的顺序闭合
正确【】{} ()
错误 [{]}]
'''

class Solution:
    def is_valid(self,s):
        dic = {'(':')','[':']','{':'}'}
        stack = []
        for i in s:
            if i in dic.keys():
                stack.append(i)
            elif stack:
                if dic[stack[-1]]==i:
                    stack.pop(-1)
                else:
                    return False
            else:
                return False
        if len(stack) == 0:
            return True
        else:
            return False
s = ['[',']','{']
r = Solution().is_valid(s)
print(r)