【 腾讯精选练习 50 题】用队列实现栈

public class Stack {
  Queue<Integer> queue1 = new LinkedList<Integer>();
  Queue<Integer> queue2 = new LinkedList<Integer>();
  /**
   * Initialize your data structure here.
   * 
   * @return
   */
  public Stack() {
  }
  /** Push element x onto stack. */
  public void push(int x) {
    while (!queue1.isEmpty()) {
      queue2.add(queue1.peek());
      queue1.poll();
    }
    queue1.add(x);
    while (!queue2.isEmpty()) {
      queue1.add(queue2.peek());
      queue2.poll();
    }
  }
  public int pop() {
    int temp = queue1.peek();
    queue1.poll();
    return temp;
  }
  /** Get the top element. */
  public int top() {
    return queue1.peek();
  }
  /** Returns whether the stack is empty. */
  public boolean empty() {
    return queue1.isEmpty();
  }
}