剑指 Offer 09. 用两个栈实现队列

用两个栈实现一个队列。队列的声明如下，请实现它的两个函数 appendTail 和 deleteHead ，分别完成在队列尾部插入整数和在队列头部删除整数的功能。(若队列中没有元素，deleteHead 操作返回 -1 )

示例 1：

输入：
["CQueue","appendTail","deleteHead","deleteHead","deleteHead"]
[[],[3],[],[],[]]
输出：[null,null,3,-1,-1]
示例 2：

输入：
["CQueue","deleteHead","appendTail","appendTail","deleteHead","deleteHead"]
[[],[],[5],[2],[],[]]
输出：[null,-1,null,null,5,2]

class CQueue {
    //使用list模拟队列
    ArrayList<Integer> list = null;
    public CQueue() {
        list = new ArrayList<>();
    }
    
    public void appendTail(int value) {
        list.add(value);
    }
    
    public int deleteHead() {    
        if(list.size()==0) return -1;
        return list.remove(0);
    }
}

/**
 * Your CQueue object will be instantiated and called as such:
 * CQueue obj = new CQueue();
 * obj.appendTail(value);
 * int param_2 = obj.deleteHead();
 */

优化使用Linked模拟出栈入栈

class CQueue {
    LinkedList<Integer> stack1;
    LinkedList<Integer> stack2;
    public CQueue() {
        stack1 = new LinkedList<>();
        stack2 = new LinkedList<>();
    }
    //入stack1栈
    public void appendTail(int value) {
        stack1.push(value);     
    }
    //两个LinkedList模拟队列
    public int deleteHead() {
        if(stack2.isEmpty()){
            if(stack1.isEmpty())return -1;
            while(!stack1.isEmpty()){
                stack2.addFirst(stack1.pop());
            }
            return stack2.pop();
        }else{
            return stack2.pop();
        }
    }
}

/**
 * Your CQueue object will be instantiated and called as such:
 * CQueue obj = new CQueue();
 * obj.appendTail(value);
 * int param_2 = obj.deleteHead();
 */