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闯关leetcode——225. Implement Stack using Queues

news 2024/10/28 6:07:21

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题目
- 地址
- 内容
解题
- 代码地址

题目

地址

https://leetcode.com/problems/implement-stack-using-queues/

内容

Implement a last-in-first-out (LIFO) stack using only two queues. The implemented stack should support all the functions of a normal stack (push, top, pop, and empty).

Implement the MyStack class:

void push(int x) Pushes element x to the top of the stack.
int pop() Removes the element on the top of the stack and returns it.
int top() Returns the element on the top of the stack.
boolean empty() Returns true if the stack is empty, false otherwise.

Notes:

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

Example 1:

Input
[“MyStack”, “push”, “push”, “top”, “pop”, “empty”]
[[], [1], [2], [], [], []]
Output
[null, null, null, 2, 2, false]

Explanation
MyStack myStack = new MyStack();
myStack.push(1);
myStack.push(2);
myStack.top(); // return 2
myStack.pop(); // return 2
myStack.empty(); // return False

Constraints:

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

Follow-up: Can you implement the stack using only one queue?

解题

这题就是要求使用两个队列模拟栈的操作。它主要考察的是数据结构的相关知识：

队列：先进先出。
栈：先进后出。

在这里插入图片描述

#include <queue>
using namespace std;class MyStack {
public:MyStack() {}void push(int x) {q1.push(x);}int pop() {while (q1.size() > 1) {q2.push(q1.front());q1.pop();}int result = q1.front();q1 = q2;q2 = queue<int>();return result;}int top() {while (q1.size() > 1) {q2.push(q1.front());q1.pop();}int result = q1.front();q2.push(result);q1 = q2;q2 = queue<int>();return result;}bool empty() {return q1.empty();}private:queue<int> q1;queue<int> q2;
};