题目

地址

https://leetcode.com/problems/implement-queue-using-stacks/description/

内容

Implement a first in first out (FIFO) queue using only two stacks. The implemented queue should support all the functions of a normal queue (push, peek, pop, and empty).

Implement the MyQueue class:

• void push(int x) Pushes element x to the back of the queue.
• int pop() Removes the element from the front of the queue and returns it.
• int peek() Returns the element at the front of the queue.
• boolean empty() Returns true if the queue is empty, false otherwise.

Notes:

• You must use only standard operations of a stack, which means only push to top, peek/pop from top, size, and is empty operations are valid.
• Depending on your language, the stack may not be supported natively. You may simulate a stack using a list or deque (double-ended queue) as long as you use only a stack’s standard operations.

Example 1:

Input
[“MyQueue”, “push”, “push”, “peek”, “pop”, “empty”]
[[], [1], [2], [], [], []]
Output
[null, null, null, 1, 1, false]
Explanation
MyQueue myQueue = new MyQueue();
myQueue.push(1); // queue is: [1]
myQueue.push(2); // queue is: [1, 2] (leftmost is front of the queue)
myQueue.peek(); // return 1
myQueue.pop(); // return 1, queue is [2]
myQueue.empty(); // return false

Constraints:

• 1 <= x <= 9
• At most 100 calls will be made to push, pop, peek, and empty.
• All the calls to pop and peek are valid.

Follow-up: Can you implement the queue such that each operation is amortized O(1) time complexity? In other words, performing n operations will take overall O(n) time even if one of those operations may take longer.

解题

这题和《闯关leetcode——225. Implement Stack using Queues》非常类似。它要求使用两个栈表达出队列的操作。栈的特点是先进后出，队列的特点是先进先出。我们将借助一个栈来做中转。

#include <stack>
using namespace std;

class MyQueue {
public:
    MyQueue() {
        
    }
    
    void push(int x) {
        s1.push(x);
    }
    
    int pop() {
        int res = 0;
        while (!s1.empty()) {
            s2.push(s1.top());
            s1.pop();
        }
        res = s2.top();
        s2.pop();
        while (!s2.empty()) {
            s1.push(s2.top());
            s2.pop();
        }
        return res;
    }
    
    int peek() {
        int res = 0;
        while (!s1.empty()) {
            s2.push(s1.top());
            s1.pop();
        }
        res = s2.top();
        while (!s2.empty()) {
            s1.push(s2.top());
            s2.pop();
        }
        return res;
    }
    
    bool empty() {
        return s1.empty();
    }
private:
    stack<int> s1, s2;
};

代码地址

https://github.com/f304646673/leetcode/blob/main/232-Implement-Queue-using-Stacks/cplusplus/src/solution.hpp