用栈实现队列

代码简单，不在解释

package lesson01;

import java.util.Stack;

public class Code03_TwoStackQueue {

  Stack<Integer> stackPush;
  Stack<Integer> stackPop;

  public Code03_TwoStackQueue() {
    stackPush = new Stack<Integer>();
    stackPop = new Stack<Integer>();
  }

  public void push(int item) {
    stackPush.push(item);
  }

  public int poll() throws Exception {
    if (stackPush.isEmpty() && stackPop.isEmpty()) {
      throw new Exception("Queue is empty!");
    } else if (stackPop.isEmpty()) {
      while (!stackPush.isEmpty()) {
        stackPop.push(stackPush.pop());
      }
    }
    return stackPop.pop();

  }

  public int peek() throws Exception {
    if (stackPush.isEmpty() && stackPop.isEmpty()) {
      throw new Exception("Queue is empty!");
    } else if (stackPop.isEmpty()) {
      while (!stackPush.isEmpty()) {
        stackPop.push(stackPush.pop());
      }
    }
    return stackPop.peek();

  }

  public static void main(String[] args) throws Exception {
    Code03_TwoStackQueue queue = new Code03_TwoStackQueue();
    for (int i = 0; i < 7; i++) {
      queue.push(i);
    }

    for (int i = 0; i < 7; i++) {
      System.out.print(" " + queue.poll());
    }

  }

}