用队列结构实现栈

解题思路

package lesson01;

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

public class Code03_TwoQueueStack {

	Queue<Integer> queue;
	Queue<Integer> help;

	public Code03_TwoQueueStack() {
		queue = new LinkedList<Integer>();
		help = new LinkedList<Integer>();
	}

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

	public int pop() throws Exception {
		if (queue.isEmpty()) {
			throw new Exception("stack is null!");
		}
		while (queue.size() > 1) {
			help.add(queue.poll());
		}
		int res = queue.poll();
		swap();
		return res;
	}

	public int peek() throws Exception {
		if (queue.isEmpty()) {
			throw new Exception("stack is null!");
		}
		while (queue.size() > 1) {
			help.add(queue.poll());
		}
		int res = queue.poll();
		help.add(res);
		swap();
		return res;
	}

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

	public static void main(String[] args) throws Exception {
		Code03_TwoQueueStack stack = new Code03_TwoQueueStack();
		for (int i = 0; i < 7; i++) {
			stack.push(i);
		}

		for (int i = 0; i < 7; i++) {
			System.out.print(" " + stack.peek());
			stack.pop();
		}
	}

}