QueueWithTwoStacks.java

package com.coding.basic.queue;

import java.util.Stack;

/**
 * 用两个栈来实现一个队列
 * @author liuxin
 *
 * @param <E>
 */
public class QueueWithTwoStacks<E> {
	private Stack<E> stack1;    
    private Stack<E> stack2;    
    
    public QueueWithTwoStacks() {
        stack1 = new Stack<E>();
        stack2 = new Stack<E>();
    }
    
    public boolean isEmpty() {
        return stack1.isEmpty() && stack2.isEmpty();
    }
    
    public int size() {
        return stack1.size() + stack2.size();   
    }
    
    public void enQueue(E item) {
        stack1.push(item);
    }
    
    public E deQueue() {
    	if(this.isEmpty()){
    		return null;
    	}
    	
    	if(stack2.isEmpty()){
    		while(!stack1.isEmpty()){
    			stack2.push(stack1.pop());
    		}
    	}
    	
    	return stack2.pop();
    }
    
 }