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).

题目大意 #

题目要求用栈实现一个队列的基本操作:push(x)、pop()、peek()、empty()。

解题思路 #

按照题目要求实现即可。

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)
	}
}


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