剑指 Offer 09. 用两个栈实现队列

话不多说，先上题目

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]

来源：力扣（LeetCode）

class CQueue {

    private Stack s1 = null;
    private Stack s2 = null;

    public CQueue() {
        s1 = new Stack();
        s2 = new Stack();
    }
    
    public void appendTail(int value) {
        if(s2.size() > 0) {
            while(s2.size() > 0) {
                s1.push(s2.pop());
            }
        }
        s1.push(value);
    }
    
    public int deleteHead() {
        if(s1.size() > 0) {
            while(s1.size() > 0) {
                s2.push(s1.pop());
            }
        }
        if(s2.size() > 0) {
            return (int) s2.pop();
        }
        return -1;
    }
}

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