队列实现栈\栈实现队列

两个队列一个栈

public static class TwoQueuesStack {
        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;
        }

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

    }

}

两个栈一个队列

push栈

pop栈

pop栈中非空，push不可入pop栈

pop栈为空，push倒入pop直到push为空。

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();
        }
    }