0232. Implement Queue using Stacks

Implement the following operations of a queue using stacks.

push(x) – Push element x to the back of queue. pop() – Removes the element from in front of queue. peek() – Get the front element. empty() – Return whether the queue is empty. Example:

MyQueue queue = new MyQueue();

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

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, stack may not be supported natively. You may simulate a stack by using a list or deque (double-ended queue), as long as you use only standard operations of a stack. You may assume that all operations are valid (for example, no pop or peek operations will be called on an empty queue).

思路

根据题意，使用栈来实现一个队列。先进后出实现先进先出。

这里有提示，如果使用的语言没有实现Stack，就用队列。Python如果使用list来实现队列就非常的简单(算作弊)

这题的基本思路就是使用两个栈，但是具体的操作需要看实现。

可以在入队时变为FIFO，也可以在出队时变为FIFO。

入队如果变FIFO,那么在新进元素的时需要放在栈底，我们只能用另外一个栈暂存元素，放置之后再放回来。

时间复杂度 O(N)
空间复杂度 O(1)

出队如何变FIFO，那么使用另外一个栈暂存元素，把栈底元素弹出。

时间复杂度 O(1) 最坏O(N)
空间复杂度 O(1)

代码

struct MyQueue { vec1: Vec, vec2: Vec, }

/**

&self means the method takes an immutable reference.
If you need a mutable reference, change it to &mut self instead. */ impl MyQueue {

/** Initialize your data structure here. */ fn new() -> Self {

MyQueue { vec1: Vec::new(), vec2: Vec::new(), } }

/** Push element x to the back of queue. */ fn push(&mut self, x: i32) {

while let Some(v) = self.vec1.pop() { self.vec2.push(v); } self.vec2.push(x); while let Some(v) = self.vec2.pop(){ self.vec1.push(v); } }

/** Removes the element from in front of queue and returns that element. */ fn pop(&mut self) -> i32 {

self.vec1.pop().unwrap() }

/** Get the front element. */ fn peek(&self) -> i32 { *self.vec1.last().unwrap()

}

/** Returns whether the queue is empty. */ fn empty(&self) -> bool { self.vec1.is_empty()

} }

执行用时: 0 ms
内存消耗: 2.1 MB

题型与相似题

题型

1.栈 2.队列

相似题

代码链接

implement_queue_using_stacks