20. 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.

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也是上古时期做的题了……其实现在要做就是一个stack的事，看到左括号就入栈，右括号就出栈，看是不是相对应的就好。以前的这种方法可以弃了。

public class Solution {<span style="white-space:pre"></span>public boolean isValid(String s) {<span style="white-space:pre"></span>String str = "";<span style="white-space:pre"></span>int len = s.length();<span style="white-space:pre"></span>for (int i = 0; i < len; i++) {<span style="white-space:pre"></span>// if(str.length()==0&&(s.charAt(i)==')'||s.charAt(i)=='}'||s.charAt(i)==']'))return<span style="white-space:pre"></span>// false;<span style="white-space:pre"></span>if (s.charAt(i) == '(')<span style="white-space:pre"></span>str += '(';<span style="white-space:pre"></span>if (s.charAt(i) == ')') {<span style="white-space:pre"></span>if (str.length() == 0 || str.charAt(str.length() - 1) != '(')<span style="white-space:pre"></span>return false;<span style="white-space:pre"></span>else<span style="white-space:pre"></span>str = str.substring(0, str.length() - 1);<span style="white-space:pre"></span>}<span style="white-space:pre"></span>if (s.charAt(i) == '[')<span style="white-space:pre"></span>str += '[';<span style="white-space:pre"></span>if (s.charAt(i) == ']') {<span style="white-space:pre"></span>if (str.length() == 0 || str.charAt(str.length() - 1) != '[')<span style="white-space:pre"></span>return false;<span style="white-space:pre"></span>else<span style="white-space:pre"></span>str = str.substring(0, str.length() - 1);<span style="white-space:pre"></span>}<span style="white-space:pre"></span>if (s.charAt(i) == '{')<span style="white-space:pre"></span>str += '{';<span style="white-space:pre"></span>if (s.charAt(i) == '}') {<span style="white-space:pre"></span>if (str.length() == 0 || str.charAt(str.length() - 1) != '{')<span style="white-space:pre"></span>return false;<span style="white-space:pre"></span>else<span style="white-space:pre"></span>str = str.substring(0, str.length() - 1);<span style="white-space:pre"></span>}<span style="white-space:pre"></span>}<span style="white-space:pre"></span>if (str.length() == 0)<span style="white-space:pre"></span>return true;<span style="white-space:pre"></span>return false;<span style="white-space:pre"></span>}}

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