225. Implement Stack using Queues #

Problem Statement #

Implement a last-in-first-out (LIFO) stack using only two queues. The implemented stack should support all the functions of a normal stack (push, top, pop, and empty).

Implement the MyStack class:

• void push(int x) Pushes element x to the top of the stack.
• int pop() Removes the element on the top of the stack and returns it.
• int top() Returns the element on the top of the stack.
• boolean empty() Returns true if the stack is empty, false otherwise.

Notes:

• You must use only standard operations of a queue, which means that only push to back, peek/pop from front, size and is empty operations are valid.
• Depending on your language, the queue may not be supported natively. You may simulate a queue using a list or deque (double-ended queue) as long as you use only a queue's standard operations.

Example 1:

Input
["MyStack", "push", "push", "top", "pop", "empty"]
[[], [1], [2], [], [], []]
Output
[null, null, null, 2, 2, false]

Explanation
MyStack myStack = new MyStack();
myStack.push(1);
myStack.push(2);
myStack.top(); // return 2
myStack.pop(); // return 2
myStack.empty(); // return False

Constraints:

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

Follow-up: Can you implement the stack using only one queue?

Solution: #

package main

type MyStack struct {
	queue []int
}

func Constructor() MyStack {
	return MyStack{}
}

// Push x onto stack.
func (this *MyStack) Push(x int) {
	this.queue = append(this.queue, x)
}

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

// Get the top element.
func (this *MyStack) Top() int {
	if len(this.queue) == 0 {
		return 0
	}
	return this.queue[len(this.queue)-1]
}

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

... #