LeetCode(20) Valid Parentheses

简单的括号匹配题，用栈即可解决。

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下附AC代码。

class Solution:
    def isValid(self, s):
        """
        :type s: str
        :rtype: bool
        """
        bracket={')':'(','}':'{',']':'['}
        stack = ['' for i in range(len(s))]
        top=0
        for i in s:
            if i=='(' or  i=='{'or i== '[' :
                stack[top]=i
                top+=1
            else:
                if top-1<0:return False
                if stack[top-1] == bracket[i]:
                    top-=1
                else: return False
        if top != 0:
            return False
        else:
            return True