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 )

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

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

class CQueue {
private: 
    stack<int> instack;
    stack<int> outstack;/*一个作为输入栈，一个作为输出栈*/
public:
    CQueue() {

    }
    
    void appendTail(int value) {
        instack.push(value);
    }
    
    int deleteHead() {
        if(!outstack.empty()){
            int value = outstack.top();
            outstack.pop();
            return value;
        }
        else{
            while(!instack.empty()){
                int top = instack.top();
                instack.pop();
                outstack.push(top);
            }
            if(outstack.empty()){
                return -1;
            }else{
                int output = outstack.top();
                outstack.pop();
                return output;
            }
        }
    }
};

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