栈和队列——栈和队列相互实现

如何仅用栈结构实现队列结构？ 

解：准备两个栈，一个stackIn 一个stackOut，stackIn用来接收入队的元素，stackOut用来进行出队列的操作，当stackOut为空时，从stackIn处弹入（全部）即可

public class StackToQueue{
        private Stack<Integer> stackIn;
        private Stack<Integer> stackOut;

        public StackToQueue() {
            stackIn = new Stack<>();
            stackOut = new Stack<>();
        }

        public void offer(int num){
            stackIn.push(num);
        }
        public int poll(){
            if(stackOut.empty() && stackIn.empty()){
                throw new RuntimeException( "queue is empty" );
            }
            if(stackOut.empty()){
                while(!stackIn.empty()){
                    stackOut.push(stackIn.pop());
                }
            }
            return stackOut.pop();
        }

        public int peek(){
            if(stackOut.empty() && stackIn.empty()){
                throw new RuntimeException( "queue is empty" );
            }
            if(stackOut.empty()){
                while(!stackIn.empty()){
                    stackOut.push( stackIn.pop() );
                }
            }
            return stackOut.peek();
        }

        public boolean empty(){
            if(stackOut.empty() && stackIn.empty()) return false;
            return true;
        }
    }

　　

如何仅用队列结构实现栈结构？

解：准备两个队列Queue<Integer> queue = new LinkedList<>()，和help，

queue负责压入栈中的数值，当要pop()弹出数据时，将queue中的数据全部出队列到help，只剩下最后一个即可，peek也是，最后交换queue和help两个Queue指向的队列即可

public class QueueToStack{
        private Queue<Integer> queue;
        private Queue<Integer> help;

        public QueueToStack() {
            this.queue = new LinkedList<>();
            this.help = new LinkedList<>();
        }

        public void push(int num){
            queue.offer(num);
        }

        public int pop(){
            if(queue.isEmpty()){
                throw new RuntimeException( "stack is empty" );
            }
            while(queue.size() > 1){
                help.offer(queue.poll());
            }
            //此时queue.poll()之后，queue为空
            int num = queue.poll();
            swap(queue, help);
            return num;
        }

        public int peek(){
            if(queue.isEmpty()){
                throw new RuntimeException( "stack is empty" );
            }
            while(queue.size() > 1){
                help.offer(queue.poll());
            }
            int num = queue.poll();
            help.offer(num);
            swap(queue, help);
            return num;
        }

        public boolean empty(){
            if(queue.isEmpty()) return true;
            return false;
        }

        public void swap(Queue<Integer> q1, Queue<Integer> q2){
            Queue<Integer> temp = q1;
            q1 = q2;
            q2 = temp;
        }
    }