Sword finger offer 09 Implementing queues with two stacks

Use two stacks to implement a queue . The declaration of the queue is as follows , Please implement its two functions appendTail and deleteHead , The functions of inserting integers at the end of the queue and deleting integers at the head of the queue are respectively completed .( If there are no elements in the queue ,deleteHead  Operation return -1 )

Example 1：

Input ：
["CQueue","appendTail","deleteHead","deleteHead"]
[[],[3],[],[]]
Output ：[null,null,3,-1]
Example 2：

Input ：
["CQueue","deleteHead","appendTail","appendTail","deleteHead","deleteHead"]
[[],[],[5],[2],[],[]]
Output ：[null,-1,null,null,5,2]

Ideas ：

A stack is responsible for putting elements into the queue , A stack is responsible for the element out of the queue .

class CQueue {
public:
    stack<int> s1;
    stack<int> s2;
    CQueue() {
        
    }
    
    void appendTail(int value) {
        s1.push(value);
    }
    
    int deleteHead() {
        if (s1.empty() && s2.empty()) {
            return -1;
        } else if (s2.empty() && !s1.empty()) {
            while (!s1.empty()) {
                s2.push(s1.top());
                s1.pop();
            }
        }
        int tmp = s2.top();
        s2.pop();
        return tmp;
    }
};