闯关leetcode——225. Implement Stack using Queues
大纲
- 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。
#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;
};
代码地址
https://github.com/f304646673/leetcode/blob/main/225-Implement-Stack-Using-Queues/cplusplus/src/solution.hpp