leetcode栈之用两个栈实现队列

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"]
[[],[3],[],[]]
输出：[null,null,3,-1]

示例 2：

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

提示：

    1 <= values <= 10000
    最多会对 appendTail、deleteHead 进行 10000 次调用

来源：力扣（LeetCode）
链接：https://leetcode-cn.com/problems/yong-liang-ge-zhan-shi-xian-dui-lie-lcof
著作权归领扣网络所有。商业转载请联系官方授权，非商业转载请注明出处。

class CQueue {

    Stack<Integer> in;
    Stack<Integer> out;

    public CQueue() {
        in = new Stack<>();
        out = new Stack<>();
    }

    public void appendTail(int value) {
        while(!out.isEmpty()){
            in.push(out.pop());
        }
        in.push(value);
    }

    public int deleteHead() {
        while(!in.isEmpty()){
            out.push(in.pop());
        }
        if(out.isEmpty()){
            return -1;
        }
        return out.pop();
    }
}

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