题解 | #完成括号匹配#

import java.util.*;

// 注意类名必须为 Main, 不要有任何 package xxx 信息
public class Main {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        // 注意 hasNext 和 hasNextLine 的区别
        while (in.hasNextLine()) { // 注意 while 处理多个 case
            String a = in.nextLine();
            Deque<Character> deque = new LinkedList<Character>();
            Stack<Character> stack = new Stack<Character>();
            for(int i = 0;i<a.length();i++){
                if(a.charAt(i)=='['){
                    stack.add('[');
                    deque.addLast('[');
                }else{
                    if(!stack.isEmpty()){
                        stack.pop();
                        deque.addLast(']');
                    }else{
                        deque.addFirst('[');
                        deque.addLast(']');
                    }
                }
            }
            int len1 = deque.size();
            for(int i = 0;i<len1;i++)   System.out.print(deque.pollFirst());
            int len = stack.size();

            for(int i = 0;i<len;i++)    System.out.print(']');
        }
    }
}

用双向队列存答案，stack存左括号匹配即可。