[TOC]

由两个栈组成的队列

题目：

编写一个类，用两个栈实现队列，支持队列的基本操作(add,poll,peek)

题解思路：

题解代码：

import java.util.Stack;

public class Problem_02_TwoStacksImplementQueue {

  public static class TwoStacksQueue {
    public Stack<Integer> stackPush;
    public Stack<Integer> stackPop;

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

    public void add(int pushInt) {
      stackPush.push(pushInt);
    }

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

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

  public static void main(String[] args) {
    TwoStacksQueue test = new TwoStacksQueue();
    test.add(1);
    test.add(2);
    test.add(3);
    System.out.println(test.peek());
    System.out.println(test.poll());
    System.out.println(test.peek());
    System.out.println(test.poll());
    System.out.println(test.peek());
    System.out.println(test.poll());
  }

}