剑指 Offer 09：用两个栈实现队列

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 {
public:
    stack<int> st1;
    stack<int> st2;
    CQueue() {
    }
    void appendTail(int value) {
        while(!st1.empty()){
            st2.push(st1.top());
            st1.pop();
        }
        st1.push(value);
        while(!st2.empty()){
            st1.push(st2.top());
            st2.pop();
        }
    }
    int deleteHead() {
        if(st1.empty()) return -1;
        int num=st1.top();
        st1.pop();
        return num;
    }
};