解题思路

首先将N个元素入第一个队列，然后弹出N-个元素到第二个队列，然后输出第一个队列的最后一个元素，如此往复循环

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();
    }
  }

}