劍指 Offer 09. 用兩個棧實現隊列(Leetcode每日一題-2020.06.30）

Problem

用兩個棧實現一個隊列。隊列的聲明如下，請實現它的兩個函數 appendTail 和 deleteHead ，分別完成在隊列尾部插入整數和在隊列頭部刪除整數的功能。(若隊列中沒有元素，deleteHead 操作返回 -1 )

Example1

輸入：
[“CQueue”,“appendTail”,“deleteHead”,“deleteHead”]
[[],[3],[],[]]
輸出：[null,null,3,-1]

Example2

輸入：
[“CQueue”,“deleteHead”,“appendTail”,“appendTail”,“deleteHead”,“deleteHead”]
[[],[],[5],[2],[],[]]
輸出：[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();
 */