Stack simulation queue

queue (Queue) Characteristics ： fifo   FIFO   Stack (Stack) Characteristics ： First in, then out FILO

To use a stack to simulate a queue , You need to use two stack spaces to store data , data structure as follows ：

  Because the characteristic of stack is First in, then out , The characteristics of the queue are fifo , So we need to Define two stacks , Stack （in） Used for The team operation , Stack （out） Used for Out of the team operation . First put the data that needs to be queued into the stack （in）, The first data entered is stored at the bottom of the stack , The last entered data is stored at the top of the stack , To get out of the team, you need to stack （in） Data in the stack again （out） So that the first stack （in） The data is on the stack （out） The position in the is the top of the stack , The data that is finally put into the stack is on the stack （out） At the bottom of the stack , Finally on the stack （out） Carry out stack out operation , You can realize the queue out operation .  

  In the process of entering and leaving the team , What we should pay attention to is ： When you join the team , Need to judge stack （out） Is it empty , If it is empty , Directly on the stack （in） Just join the team , If not be empty , You also need to stack （out） Store the data in the stack first （in） Inside , In the process of joining the team . When leaving the team, you need to judge the stack （in） Is it empty , If it is empty , Directly on the stack （out） Just go out of the queue , If not be empty , You also need to stack （in） Store the data in the stack first （out） Inside , Out of line operation .

Code implementation ：

public class Main {
	public static void main(String[] args) {
		MyQueue<String> queue = new MyQueue<String>();
		queue.offer("A1");
		System.out.println(queue.poll()+" Out of the team ");
		queue.offer("A2");
		System.out.println(queue.poll()+" Out of the team ");
		System.out.println(queue);
        // Traverse MyQueue
		while(!queue.isEmpty()) {
			System.out.println(queue.poll());
		}
	}
}
//  Stack simulation queue 
class MyQueue<E>{
	
	private Stack<E> in = new Stack<E>();
	private Stack<E> out = new Stack<E>();
 	//  The team 
	public void offer(E e) {
		while(!out.isEmpty()) {
			in.push(out.pop());
		}
		// Add a new element 
		in.push(e);
	}
	// Out of the team 
	public E poll() {
		while(!in.isEmpty()) {
			out.push(in.pop());
		}
		return out.pop();
	}
	// Determines if the queue is empty 
	public boolean isEmpty() {
		if(in.size()==0 && out.size()==0) {
			return true;
		}
		return false;
	}
}