0232. Implement Queue Using Stacks

# 232. Implement Queue using Stacks#

## 题目 #

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

``````

Note:

• 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).

## 代码 #

``````
package leetcode

type MyQueue struct {
Stack *[]int
Queue *[]int
}

/** Initialize your data structure here. */
func Constructor232() MyQueue {
tmp1, tmp2 := []int{}, []int{}
return MyQueue{Stack: &tmp1, Queue: &tmp2}
}

/** Push element x to the back of queue. */
func (this *MyQueue) Push(x int) {
*this.Stack = append(*this.Stack, x)
}

/** Removes the element from in front of queue and returns that element. */
func (this *MyQueue) Pop() int {
if len(*this.Queue) == 0 {
this.fromStackToQueue(this.Stack, this.Queue)
}

popped := (*this.Queue)[len(*this.Queue)-1]
*this.Queue = (*this.Queue)[:len(*this.Queue)-1]
return popped
}

/** Get the front element. */
func (this *MyQueue) Peek() int {
if len(*this.Queue) == 0 {
this.fromStackToQueue(this.Stack, this.Queue)
}

return (*this.Queue)[len(*this.Queue)-1]
}

/** Returns whether the queue is empty. */
func (this *MyQueue) Empty() bool {
return len(*this.Stack)+len(*this.Queue) == 0
}

func (this *MyQueue) fromStackToQueue(s, q *[]int) {
for len(*s) > 0 {
popped := (*s)[len(*s)-1]
*s = (*s)[:len(*s)-1]

*q = append(*q, popped)
}
}

`````` Apr 8, 2023 Edit this page