题目

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.

题解:

这道题是一道很常见的老题,记得当时看严蔚敏的数据结构还有数据结构课上面都见过这道题,一道训练栈的基础题。

解题方法是:

一个个检查给的characters,如果是左括号都入栈;如果是右括号,检查栈如果为空,证明不能匹配,如果栈不空,弹出top,与当前扫描的括号检查是否匹配。

全部字符都检查完了以后,判断栈是否为空,空则正确都匹配,不空则证明有没匹配的。

注意:

检查字符是用==,检查String是用.isEqual(),因为String是引用类型,值相等但是地址可能不等。

代码如下:

 1     public boolean isValid(String s) {
 2         if(s.length()==0||s.length()==1)
 3             return false;
 4 
 5         Stack<Character> x = new Stack<Character>();
 6         for(int i=0;i<s.length();i++){
 7             if(s.charAt(i)=='('||s.charAt(i)=='{'||s.charAt(i)=='['){
 8                 x.push(s.charAt(i));
 9             }else{
                 if(x.size()==0)
                     return false;
                 char top = x.pop();
                 if(s.charAt(i)==')')
                     if(top!='(')
                         return false;
                 else if(s.charAt(i)=='}')
                     if(top!='{')
                         return false;
                 else if(s.charAt(i)==']')
                     if(top!='[')
                         return false;
             }
     }
         return x.size()==0;
 }

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