剑指Offer - 面试题9. 用两个栈实现队列

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

class CQueue {
	int v;
    stack<int> in;
    stack<int> out;
public:
    CQueue() {}
    
    void appendTail(int value) {
        in.push(value);
    }
    
    int deleteHead() {
        if(in.empty() && out.empty())
        	return -1;
        if(out.empty() && !in.empty())
        {
        	while(!in.empty())
        	{
        		out.push(in.top());
        		in.pop();
        	}
        }
        v = out.top();
    	out.pop();
    	return v;
    }
};