【LeetCode】232. 用栈实现队列

ListNode链接:https://leetcode-cn.com/problems/implement-queue-using-stacks/

使用栈实现队列的下列操作：

• push(x) -- 将一个元素放入队列的尾部。
• pop() -- 从队列首部移除元素。
• peek() -- 返回队列首部的元素。
• empty() -- 返回队列是否为空。

示例:

MyQueue queue = new MyQueue();

queue.push(1);
queue.push(2);  
queue.peek();  // 返回 1
queue.pop();   // 返回 1
queue.empty(); // 返回 false

class MyQueue {
public:
    /** Initialize your data structure here. */
    MyQueue() {
        
    }
    
    /** Push element x to the back of queue. */
    void push(int x) {
        up.push(x);
    }
    
    /** Removes the element from in front of queue and returns that element. */
    int pop() {
        while(!up.empty())
        {
            out.push(up.top());
            up.pop();
        }
        int tmp = out.top();
        out.pop();
        while(!out.empty())
        {
            up.push(out.top());
            out.pop();
        }
        return tmp;
    }
    
    /** Get the front element. */
    int peek() {
        while(!up.empty())
        {
            out.push(up.top());
            up.pop();
        }
        int tmp = out.top();
        while(!out.empty())
        {
            up.push(out.top());
            out.pop();
        }
        return tmp;
    }
    
    /** Returns whether the queue is empty. */
    bool empty() {
        return up.empty();
    }
    stack<int> up;
    stack<int> out;
};

/**
 * Your MyQueue object will be instantiated and called as such:
 * MyQueue* obj = new MyQueue();
 * obj->push(x);
 * int param_2 = obj->pop();
 * int param_3 = obj->peek();
 * bool param_4 = obj->empty();
 */