Leetcode|入栈+出栈实现队列|剑指 Offer 09. 用两个栈实现队列

class CQueue {
public:
    stack<int> stk1, stk2;   // stk1用于入队，stk2用于出队
    CQueue() {
        while (!stk1.empty()) stk1.pop();
        while (!stk2.empty()) stk2.pop();
    }
    
    void appendTail(int value) {
        stk1.push(value);   // 入队直接压栈stk1即可
    }
    
    int deleteHead() {
        int res;
        if (stk2.empty()) {
            // 若两栈均为空，则说明队列为空，返回-1即可
            if (stk1.empty()) return -1;
            // 若用于出队的stk2为空且用于入队的stk1不为空，则把stk1中的元素逐个出栈倒入stk2中
            while (!stk1.empty()) {
                stk2.push(stk1.top());
                stk1.pop();
            }
        } 
        // stk2出栈元素即为队列对应删除元素
        res = stk2.top();
        stk2.pop();
        return res;
    }
};

/*
int main(){
    CQueue* q = new CQueue();
    for (int i = 3; i <= 6; i++)
        q->appendTail(i);
    for (int i = 0; i <= 6; i++)
        cout << q->deleteHead() << endl;
    return 0;
}
*/