剑指 Offer 09. 用两个栈实现队列

class CQueue {
public:
    stack<int>s1,s2;
    CQueue() {
        while(!s1.empty()){
            s1.pop();
        }
        while(!s2.empty()){
            s2.pop();
        }

    }
    
    void appendTail(int value) {
        s1.push(value);
        return;

    }
    
    int deleteHead() {
        if(s2.empty()){
            while(!s1.empty()){
                s2.push(s1.top());
                s1.pop();
           
            }
        }
        if( s2.empty()){
            return -1;
        }
        else{
           int elem = s2.top();
           s2.pop();
           return elem;
        }
    }
};