Leet Code OJ 20. Valid Parentheses [Difficulty: Easy]

题目：
Given a string containing just the characters , determine if the input string is valid.

The brackets must close in the correct order, “()” and “()[]{}” are all valid but “(]” and “([)]” are not.

翻译：
给定一个字符串，只包含’(‘, ‘)’, ‘{‘, ‘}’, ‘[’ 和’]’这些字符，检查它是否是“有效”的。
括号必须以正确的顺序关闭，例如”()” 和”()[]{}”都是有效的，”(]” 和”([)]”是无效的。

分析：
本题考查的是栈结构，具有后进先出的特性。有效包含2个方面，第一个是如果是关闭的括号，前一位一定要刚好有一个开启的括号；第二个是最终结果，需要把所有开启的括号都抵消完。一个比较容易出错的地方是，遇到关闭括号时，要先判断栈是否已经空了。

Java版代码：

public class Solution {public boolean isValid(String s) {char[] charArr=s.toCharArray();List<Character> list=new ArrayList<>();for(Character c:charArr){if(c=='('||c=='{'||c=='['){list.add(c);}else{if(list.size()==0){return false;}char last=list.get(list.size()-1);if(c==')'&&last!='('){return false;}else if(c=='}'&&last!='{'){return false;}else if(c==']'&&last!='['){return false;}list.remove(list.size()-1);}}if(list.size()!=0){return false;}return true;}
}

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