232. Implement Queue using Stacks #

Problem Statement #

Implement a first in first out (FIFO) queue using only two stacks. The implemented queue should support all the functions of a normal queue (push, peek, pop, and empty).

Implement the MyQueue class:

• void push(int x) Pushes element x to the back of the queue.
• int pop() Removes the element from the front of the queue and returns it.
• int peek() Returns the element at the front of the queue.
• boolean empty() Returns true if the queue is empty, false otherwise.

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, the stack may not be supported natively. You may simulate a stack using a list or deque (double-ended queue) as long as you use only a stack's standard operations.

Example 1:

Input
["MyQueue", "push", "push", "peek", "pop", "empty"]
[[], [1], [2], [], [], []]
Output
[null, null, null, 1, 1, false]

Explanation
MyQueue myQueue = new MyQueue();
myQueue.push(1); // queue is: [1]
myQueue.push(2); // queue is: [1, 2] (leftmost is front of the queue)
myQueue.peek(); // return 1
myQueue.pop(); // return 1, queue is [2]
myQueue.empty(); // return false

Constraints:

• 1 <= x <= 9
• At most 100 calls will be made to push, pop, peek, and empty.
• All the calls to pop and peek are valid.

Follow-up: Can you implement the queue such that each operation is amortized O(1) time complexity? In other words, performing n operations will take overall O(n) time even if one of those operations may take longer.

Solution: #

package main

type MyQueue struct {
	stack []int
}

func Constructor() MyQueue {
	return MyQueue{}
}

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

// Removes and returns the element at the front of queue.
func (this *MyQueue) Pop() int {
	if len(this.stack) == 0 {
		return 0
	}
	pop := this.stack[0]
	this.stack = this.stack[1:]
	return pop
}

// Get the front element.
func (this *MyQueue) Peek() int {
	if len(this.stack) == 0 {
		return 0
	}
	return this.stack[0]
}

// Return whether the queue is empty.
func (this *MyQueue) Empty() bool {
	return len(this.stack) == 0
}

... #