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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