用两个栈实现队列 

力扣（LeetCode）官网 - 全球极客挚爱的技术成长平台

    class CQueue {
        Stack<Integer>in=new Stack<>();
        Stack<Integer>out=new Stack<>();
        public CQueue() {
            Stack<Integer>in=new Stack<>();
            Stack<Integer>out=new Stack<>();
        }

        public void appendTail(int value) {
            while(!out.isEmpty())
            {
                int num=out.pop();
                in.push(num);
            }
            in.push(value);
        }

        public int deleteHead() {
            while(!in.isEmpty())
            {
                int num=in.pop();
                out.push(num);
            }
            if(out.isEmpty())return -1;
            return out.pop();
        }
    }

用队列实现栈

力扣（LeetCode）官网 - 全球极客挚爱的技术成长平台

    class MyStack {

        LinkedList<Integer>list=new LinkedList<>();
        public MyStack() {
            LinkedList<Integer>list=new LinkedList<>();
        }

        public void push(int x) {
            list.push(x);
        }

        public int pop() {
           return list.pop();
        }

        public int top() {
           return list.peek();
        }

        public boolean empty() {
            return list.isEmpty();
        }
    }

力扣（LeetCode）官网 - 全球极客挚爱的技术成长平台

class MyQueue {
        Stack<Integer>in=new Stack<>();
        Stack<Integer>out=new Stack<>();
        public MyQueue() {
            Stack<Integer>in=new Stack<>();
            Stack<Integer>out=new Stack<>();
        }

        public void push(int x) {
            while(!out.isEmpty())
            {
                int num=out.pop();
                in.push(num);
            }
            in.push(x);
        }

        public int pop() {
            while(!in.isEmpty())
            {
                int num=in.pop();
                out.push(num);
            }
            if(out.isEmpty())return -1;
            return out.pop();
        }

        public int peek() {
            while(!in.isEmpty())
            {
                int num=in.pop();
                out.push(num);
            }
            if(out.isEmpty())return -1;
            return out.peek();
        }

        public boolean empty() {
            if(in.isEmpty()&&out.isEmpty())return true;
            return false;
        }
    }