问题

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 )

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

示例

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

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

解答

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

重点思路

使用两个栈，一个栈用于输入，一个栈用于输出，具体见图：