双栈实现队列

class CQueue {
public:
    stack<int> s1;
    stack<int> s2;
    

    CQueue() {
        while(!s1.empty()) s1.pop();
        while(!s2.empty()) s2.pop();
    }
    
    void appendTail(int value) {
        s1.push(value);
    }
    
    int deleteHead() { //只有s2空的时候才把s1的东西倒腾过去
    //不需要每次都来回倒腾
        if(s2.empty()){
            while(!s1.empty()) {
            s2.push(s1.top());
            s1.pop();
            }
        }
        if(s2.empty()) return -1;
        int temp = s2.top();
        s2.pop();
        return temp;
    }
};

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