队列实现栈

1.用两个队列实现一个栈
2.一个队列只留一个元素，另一个队列放其余元素

public static class TwoQueuesStack {
 //申请两个队列
        private Queue<Integer> queue;
        private Queue<Integer> help;

        public TwoQueuesStack() {
            queue = new LinkedList<Integer>();
            help = new LinkedList<Integer>();
        }

        public void push(int pushInt) {
            queue.add(pushInt);
        }

        public int peek() {
            if (queue.isEmpty()) {
                throw new RuntimeException("Stack is empty!");
            }
            while (queue.size() != 1) {
                help.add(queue.poll());
            }
            int res = queue.poll();
            help.add(res);
            swap();
            return res;
        }

        public int pop() {
            if (queue.isEmpty()) {
                throw new RuntimeException("Stack is empty!");
            }
            while (queue.size() != 1) {
                help.add(queue.poll());
            }
            int res = queue.poll();
            swap();
            return res;
        }

        //交换
        private void swap() {
            Queue<Integer> tmp = help;
            help = queue;
            queue = tmp;
        }

    }

栈实现队列

1.首先考虑2个结构的特点
2.一个栈只进，一个栈只出

public static class TwoStacksQueue {
        private Stack<Integer> stackPush;
        private Stack<Integer> stackPop;

        public TwoStacksQueue() {
            stackPush = new Stack<Integer>();
            stackPop = new Stack<Integer>();
        }

        public void push(int pushInt) {
            stackPush.push(pushInt);
        }

        public int poll() {
            if (stackPop.empty() && stackPush.empty()) {
                throw new RuntimeException("Queue is empty!");
            } else if (stackPop.empty()) {
                while (!stackPush.empty()) {
                    stackPop.push(stackPush.pop());
                }
            }
            return stackPop.pop();
        }

        public int peek() {
            if (stackPop.empty() && stackPush.empty()) {
                throw new RuntimeException("Queue is empty!");
            } else if (stackPop.empty()) {
                while (!stackPush.empty()) {
                    stackPop.push(stackPush.pop());
                }
            }
            return stackPop.peek();
        }
    }