Easy-题目47：20. Valid Parentheses

题目原文：
Given a string containing just the characters ‘(‘, ‘)’, ‘{‘, ‘}’, ‘[’ and ‘]’, determine if the input string is valid.
The brackets must close in the correct order, “()” and “()[]{}” are all valid but “(]” and “([)]” are not.
题目大意：
给出一个括号字符串，判断括号是否正确匹配。
题目分析：
本题考查堆栈的使用，遇到左边的括号全部入栈，遇到右边的括号则看栈顶的括号是不是对应的左括号，如是则弹出，如不是则返回错误。如果整个括号字符串扫完之后栈恰好为空，则是合法匹配。
源码：（language：python）

class Solution(object):
    def isValid(self, s):
        """
        :type s: str
        :rtype: bool
        """
        stack=[]
        for c in s:
            if c in ['(','[','{']:
                stack.append(c)
            else:
                if(len(stack)==0):
                    return False
                top=stack[len(stack)-1]
                if ((c == ')' and top=='(') or (c == ']' and top=='[') or (c == '}' and top == '{')):
                    stack.pop()
                else:
                    return False

        return len(stack)==0

成绩：
43ms,beats 55.57%,众数40ms,27.91%