LintCode Python 简单级题目 423.有效的括号序列

class Solution:
    # @param {string} s A string
    # @return {boolean} whether the string is a valid parentheses
    def isValidParentheses(self, s):
        # Write your code here
        if s is None: return False
        
        x = [‘[‘,‘(‘,‘{‘]
        y = ["]",")","}"]
        z = ["()","[]","{}"]
        
        res = []
        for i in s:
            if i in x:
                res.append(i) # 入栈
            elif i in y:
                # 如果收到一个右括号，但是res中无左括号，直接返回False
                if res == []:
                    return False
                else:
                    temp = res.pop(-1) + i
                    # 其余情况，出栈一个左括号，判断左括号+右括号是否有效
                    if temp not in z:
                        return False
        # 如果所有括号对都满足，但是res还有左括号，返回False
        if len(res) != 0:
            return False
        return True