队列实现栈与栈实现队列

队列实现栈结构:使用两个队列,入栈时进入队列1,出栈时将队列1的前n-1个元素压入队列2中,弹出最后一个元素,然后交换队列1与队列2的角色.

import java.util.LinkedList;
import java.util.Queue;

public class TwoQueueStack {
	private Queue<Integer> data;
	private Queue<Integer> help;
	
	public TwoQueueStack() {
		data = new LinkedList<Integer>();
		help = new LinkedList<Integer>();
	}
	
	public void push(int pushInt) {
		data.add(pushInt);
	}
	
	public int peek() {
		if(data.isEmpty()) {
			throw new RuntimeException("Stack is empty!");
		}
		while(data.size() != 1) {
			help.add(data.poll());
		}
		int res = data.poll();
		help.add(res);
		swap();
		return res;
	}
	
	public int pop() {
		if(data.isEmpty()) {
			throw new RuntimeException("Stack is empty!");
		}
		while(data.size() > 1) {
			help.add(data.poll());
		}
		int res = data.poll();
		swap();
		return res;
	}
	
	private void swap() {
		Queue<Integer> tmp = help;
		help = data;
		data = tmp;
	}
	public static void main(String[] args) {
		TwoQueueStack stack = new TwoQueueStack();
		stack.push(6);
		stack.push(8);
		System.out.println(stack.pop());
		System.out.println(stack.peek());
	}
}

运行结果:

栈实现队列结构:进队时将元素压入栈1,然后将栈1元素全部压入栈2(但只有在栈2为空时才执行),出队时取栈2栈顶元素即可.

import java.util.Stack;
public class TwoStackQueue {
	private Stack<Integer> stackPush;
	private Stack<Integer> stackPop;
	
	public TwoStackQueue() {
		stackPush = new Stack<Integer>();
		stackPop = new Stack<Integer>();
	}
	
	public void push(int pushInt) {
		stackPush.push(pushInt);
	}
	
	public int poll() {
		if(stackPush.isEmpty() && stackPop.isEmpty()) {
			throw new RuntimeException("Queue is empty!");
		}
		pour();
		return stackPop.pop();
	}
	
	public int peek() {
		if(stackPop.isEmpty() && stackPush.isEmpty()) {
			throw new RuntimeException("Queue is empty!");
		}
		pour();
		return stackPop.peek();
	}
	
	public void pour() {
		if(!stackPop.isEmpty()) {
			return;
		}
		while(!stackPush.isEmpty()) {
			stackPop.push(stackPush.pop());
		}
	}
	public static void main(String[] args) {
		TwoStackQueue queue = new TwoStackQueue();
		queue.push(7);
		queue.push(4);
		System.out.println(queue.peek());
		System.out.println(queue.poll());
		System.out.println(queue.peek());
		queue.push(6);
		System.out.println(queue.peek());
	}
}

运行结果: