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"]
[[],[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]

class CQueue {
public:
    CQueue() {}
    
    void appendTail(int value) {
        stack1.push(value);
    }
    
    int deleteHead() {
        if(!stack2.empty())
        {
            int s = stack2.top();
            stack2.pop();
            return s;
        }
        else
        {
            while(!stack1.empty())
            {
                stack2.push(stack1.top());
                stack1.pop();
            }
            
            if(stack2.empty())
                return -1;
            int a = stack2.top();
            stack2.pop();
            return a;
        }
    }
private:
    stack<int> stack1;
    stack<int> stack2;
};

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