java 队列和栈相互实现

一、队列实现栈

public class queue2stack {

    public static void main(String[] args) {
        QS qs = new QS();
        qs.push("1");
        qs.push("2");
        qs.push("3");
        System.out.println(qs.pop());
        System.out.println(qs.pop());
        System.out.println(qs.peek());

        QS qs2 = new QS();
        qs2.push("1");
        qs2.push("2");
        qs2.push("3");
        System.out.println(qs2.pop());
        System.out.println(qs2.pop());
        System.out.println(qs2.peek());

    }

    static class QS{

        private Queue queueMain = new ArrayDeque();
        private Queue queueWork = new ArrayDeque();

        public boolean push(Object object){
            try {
                queueMain.offer(object);
                return true;
            } catch (Exception e) {
                return false;
            }
        }

        public int size() {
            return queueMain.size();
        }

        public Object pop(){
            if(queueMain.isEmpty()) {
                return null;
            }
            Queue temp = new ArrayDeque();
            int size = queueMain.size();
            for (int i = 0; i < size - 1; i ++) {
                temp.offer(queueMain.poll());
            }
            Object o = queueMain.poll();
            int size1 = temp.size();
            for (int i = 0; i < size1; i ++) {
                queueMain.offer(temp.poll());
            }
            return o;
        }

        public Object peek(){
            if(queueMain.isEmpty()) {
                return null;
            }
            Queue temp = new ArrayDeque();
            int size = queueMain.size();

            for (int i = 0; i < size - 1; i ++) {
                temp.offer(queueMain.poll());
            }
            Object o = queueMain.peek();
            temp.offer(queueMain.poll());
            int size1 = temp.size();
            for (int i = 0; i < size1; i ++) {
                queueMain.offer(temp.poll());
            }
            return o;
        }

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

        /**********优化***********/
        public boolean push2(Object object){
            try {
                if (queueMain.isEmpty() && queueWork.isEmpty()) {
                    queueMain.offer(object);
                }

                if (queueMain.isEmpty()) {
                    queueWork.offer(object);
                }

                if(queueWork.isEmpty()) {
                    queueMain.offer(object);
                }

                return true;
            } catch (Exception e) {
                return false;
            }
        }

        public Object pop2(){
            if(queueMain.isEmpty() && queueWork.isEmpty()) {
                return null;
            }

            if (queueMain.isEmpty()) {
                while (queueWork.size() > 1) {
                    queueMain.offer(queueWork.poll());
                }
                return queueWork.poll();
            }

            if (queueWork.isEmpty()) {
                while (queueMain.size() > 1) {
                    queueWork.offer(queueMain.poll());
                }
                return queueMain.poll();
            }

            return null;
        }

        public Object peek2(){
            if(queueMain.isEmpty() && queueWork.isEmpty()) {
                return null;
            }

            if (queueMain.isEmpty()) {
                while(queueWork.size() > 1) {
                    queueMain.offer(queueWork.poll());
                }
                Object e = queueWork.peek();
                queueMain.offer(queueWork.poll());
                return e;
            }

            if (queueWork.isEmpty()) {
                while(queueMain.size() > 1) {
                    queueWork.offer(queueMain.poll());
                }
                Object e = queueMain.peek();
                queueWork.offer(queueMain.poll());
                return e;
            }
            return null;
        }
    }

}    

二、栈实现队列

public class Stack2queue {

    public static void main(String[] args) {

        SQ sq = new SQ();
        sq.offer("a");
        sq.offer("b");
        sq.offer("c");
        System.out.println(sq.poll());
        System.out.println(sq.poll());
        sq.offer("d");
        sq.offer("e");
        System.out.println(sq.poll());
        System.out.println(sq.peek());
        System.out.println(sq.poll());
        System.out.println(sq.poll());

    }

    static class SQ{

        private Stack stackMain = new Stack();
        private Stack stackWork = new Stack();

        public boolean offer(Object ele) {
            stackMain.push(ele);
            return true;
        }

        public Object poll() {
            if (stackWork.empty()) {
                while(stackMain.size() > 0) {
                    stackWork.push(stackMain.pop());
                }
            }

            if (stackWork.empty()) {
                return null;
            }
            return stackWork.pop();
        }

        public Object peek() {
            if (stackWork.empty()) {
                while(stackMain.size() > 0) {
                    stackWork.push(stackMain.pop());
                }
            }

            if (stackWork.empty()) {
                return null;
            }
            return stackWork.peek();
        }
    }

}

结果自行运行测试