[Leetcode]Valid Parentheses

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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.

判断括号序列是否合法~维护一个栈，碰到左括号就入栈，碰到右括号时检查栈顶括号是否与当前右括号配对，如不配对，直接返回False；注意最后还得检查栈是否为空，如果栈不为空的话，说明还有没配对的括号，故应返回False

class Solution:    # @return a boolean    def isValid(self, s):        if s is None or len(s) == 0: return True        stack = []        for i in xrange(len(s)):            if s[i] in ['(', '[', '{']:                stack.append(s[i])            else:                if stack and ((s[i] == ')' and stack[-1] == '(') or (s[i] == ']' and stack[-1] == '[') or (s[i] ==                     '}' and stack[-1] == '{')):                    stack.pop()                else:                    return False        return len(stack) == 0

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