剑指 Offer 09. 用两个栈实现队列(Leetcode每日一题-2020.06.30）

Problem

用两个栈实现一个队列。队列的声明如下，请实现它的两个函数 appendTail 和 deleteHead ，分别完成在队列尾部插入整数和在队列头部删除整数的功能。(若队列中没有元素，deleteHead 操作返回 -1 )

Example1

输入：
[“CQueue”,“appendTail”,“deleteHead”,“deleteHead”]
[[],[3],[],[]]
输出：[null,null,3,-1]

Example2

输入：
[“CQueue”,“deleteHead”,“appendTail”,“appendTail”,“deleteHead”,“deleteHead”]
[[],[],[5],[2],[],[]]
输出：[null,-1,null,null,5,2]

Solution

class CQueue {
public:
    CQueue() {

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

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

                int tmp = s1.top();
                s1.pop();

                return tmp;
            }
        }

    }
private:
    stack<int> s1;
    stack<int> s2;
};

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