20. Valid Parentheses ★

题目内容：

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.

 

题目分析：有字符串的性质可以分析出，可以用栈来存储字符串中的每个字符，过程类似于自动机中的下推自动机。

 

题目代码：