Implement Stack Using Queues

                                            
class MyStack {
    Queue<Integer> queue;
    
    public MyStack() {
        queue = new LinkedList<>();
    }
    
    public void push(int x) {
        Queue<Integer> temp = new LinkedList<>();
        temp.add(x);
        while (!queue.isEmpty()) {
            temp.add(queue.poll());
        }
        queue = temp;
    }
    
    public int pop() {
        return queue.poll();
    }
    
    public int top() {
        return queue.peek();
    }
    
    public boolean empty() {
        return queue.isEmpty();
    }
}

                                            
class MyStack(object):
    def __init__(self):
        self.queue = []

    def push(self, x):
        self.queue.append(x)
        for _ in range(len(self.queue) - 1):
            self.queue.append(self.queue.pop(0))

    def pop(self):
        return self.queue.pop(0)

    def top(self):
        return self.queue[0]

    def empty(self):
        return len(self.queue) == 0

                                            
typedef struct {
    int front;
    int rear;
    int size;
    int* array;
} MyStack;

MyStack* myStackCreate() {
    MyStack* stack = (MyStack*)malloc(sizeof(MyStack));
    stack->front = 0;
    stack->rear = -1;
    stack->size = 0;
    stack->array = (int*)malloc(100 * sizeof(int));
    return stack;
}

void myStackPush(MyStack* obj, int x) {
    obj->rear = (obj->rear + 1) % 100;
    obj->array[obj->rear] = x;
    obj->size++;
}

int myStackPop(MyStack* obj) {
    int val = obj->array[obj->rear];
    obj->rear = (obj->rear - 1 + 100) % 100;
    obj->size--;
    return val;
}

int myStackTop(MyStack* obj) {
    return obj->array[obj->rear];
}

bool myStackEmpty(MyStack* obj) {
    return obj->size == 0;
}

void myStackFree(MyStack* obj) {
    free(obj->array);
    free(obj);
}

                                            
public class MyStack {
    private Queue<int> queue;

    public MyStack() {
        queue = new Queue<int>();
    }

    public void Push(int x) {
        queue.Enqueue(x);
        int size = queue.Count;
        while (size > 1) {
            queue.Enqueue(queue.Dequeue());
            size--;
        }
    }

    public int Pop() {
        return queue.Dequeue();
    }

    public int Top() {
        return queue.Peek();
    }

    public bool Empty() {
        return queue.Count == 0;
    }
}

                                            
var MyStack = function() {
    this.queue = [];
};

MyStack.prototype.push = function(x) {
    const tempQueue = [];
    tempQueue.push(x);
    while (this.queue.length > 0) {
        tempQueue.push(this.queue.shift());
    }
    this.queue = tempQueue;
};

MyStack.prototype.pop = function() {
    return this.queue.shift();
};

MyStack.prototype.top = function() {
    return this.queue[0];
};

MyStack.prototype.empty = function() {
    return this.queue.length === 0;
};

                                            
class MyStack {
    private queue: number[];

    constructor() {
        this.queue = [];
    }

    push(x: number): void {
        this.queue.push(x);
    }

    pop(): number {
        return this.queue.pop();
    }

    top(): number {
        return this.queue[this.queue.length - 1];
    }

    empty(): boolean {
        return this.queue.length === 0;
    }
}

                                            
class MyStack {
    private $queue;

    function __construct() {
        $this->queue = new SplQueue();
    }

    function push($x) {
        $this->queue->enqueue($x);
        $size = $this->queue->count();
        while ($size > 1) {
            $this->queue->enqueue($this->queue->dequeue());
            $size--;
        }
    }

    function pop() {
        return $this->queue->dequeue();
    }

    function top() {
        return $this->queue->bottom();
    }

    function empty() {
        return $this->queue->isEmpty();
    }
}

                                            
class MyStack {
    var queue: [Int]
    
    init() {
        queue = [Int]()
    }
    
    func push(_ x: Int) {
        queue.append(x)
        for _ in 0..<queue.count-1 {
            queue.append(queue.removeFirst())
        }
    }
    
    func pop() -> Int {
        return queue.removeFirst()
    }
    
    func top() -> Int {
        return queue.first!
    }
    
    func empty() -> Bool {
        return queue.isEmpty
    }
}

                                            
class MyStack {
  List<int> queue = [];

  void push(int x) {
    queue.add(x);
  }

  int pop() {
    int size = queue.length;
    for (int i = 0; i < size - 1; i++) {
      queue.add(queue.removeAt(0));
    }
    return queue.removeAt(0);
  }

  int top() {
    return queue.last;
  }

  bool empty() {
    return queue.isEmpty;
  }
}

                                            
type MyStack struct {
    queue []int
}

func Constructor() MyStack {
    return MyStack{}
}

func (stack *MyStack) Push(x int) {
    stack.queue = append(stack.queue, x)
    n := len(stack.queue)
    for i := 0; i < n-1; i++ {
        stack.queue = append(stack.queue, stack.queue[0])
        stack.queue = stack.queue[1:]
    }
}

func (stack *MyStack) Pop() int {
    res := stack.queue[0]
    stack.queue = stack.queue[1:]
    return res
}

func (stack *MyStack) Top() int {
    return stack.queue[0]
}

func (stack *MyStack) Empty() bool {
    return len(stack.queue) == 0
}

                                            
class MyStack
    def initialize()
        @queue = []
    end

    def push(x)
        @queue << x
    end

    def pop()
        @queue.pop
    end

    def top()
        @queue.last
    end

    def empty()
        @queue.empty?
    end
end

                                            
use std::collections::VecDeque;

struct MyStack {
    queue: VecDeque<i32>,
}

impl MyStack {
    fn new() -> Self {
        MyStack {
            queue: VecDeque::new(),
        }
    }
    
    fn push(&mut self, x: i32) {
        self.queue.push_back(x);
        let mut size = self.queue.len() as i32;
        while size > 1 {
            let front = self.queue.pop_front().unwrap();
            self.queue.push_back(front);
            size -= 1;
        }
    }
    
    fn pop(&mut self) -> i32 {
        self.queue.pop_front().unwrap()
    }
    
    fn top(&self) -> i32 {
        *self.queue.front().unwrap()
    }
    
    fn empty(&self) -> bool {
        self.queue.is_empty()
    }
}

To solve this coding challenge, we need to simulate the behavior of a LIFO (Last-In-First-Out) stack using only two queues. The challenge involves creating a class,

MyStack

, that provides the typical stack operations:

push

pop

top

, and

empty

. Each operation must be implemented only with standard queue operations: enqueue (add to back), dequeue (remove from front), size, and check for empty. Let’s start by elaborating on how to achieve this step-by-step for each method:

# Explanation

Initialization (
__init__

) :

We need to initialize two queues. For simplicity, let's name them
primaryQueue

and
secondaryQueue

. We use these two queues to manage elements in a way that simulates stack behavior.

Push Operation :

When pushing an element, we can enqueue it to the
primaryQueue

.
However, to maintain the LIFO property, we must transfer all elements to the
secondaryQueue

, push the new element to
primaryQueue

, and then return all elements back from
secondaryQueue

to
primaryQueue

.

Pop Operation :

The pop operation should remove and return the front element of
primaryQueue

because, after rearranging during the push, the front of the queue simulates the top of a stack.

Top Operation :

To get the top element, we simply return the front element of
primaryQueue

. Since the queue is already rearranged whenever we push, this will yield the correct stack top.

Empty Operation :

The stack is empty if and only if
primaryQueue

is empty. Thus, we return whether
primaryQueue

is empty.

Detailed Steps in Pseudocode

Below is the detailed pseudocode for each method:

                                            
# This is the class definition for MyStack
class MyStack:

# Constructor to initialize the queues
def initialize():
primaryQueue = []
secondaryQueue = []

# Method to push an element onto the stack
def push(element):
# Push element to secondaryQueue
secondaryQueue.enqueue(element)

# Transfer all elements from primaryQueue to secondaryQueue
while not primaryQueue.is_empty():
secondaryQueue.enqueue(primaryQueue.dequeue())

# Swap names of primaryQueue and secondaryQueue
tempQueue = primaryQueue
primaryQueue = secondaryQueue
secondaryQueue = tempQueue

# Method to pop the top element from the stack
def pop():
# If primaryQueue is not empty, dequeue and return the front element
if not primaryQueue.is_empty():
return primaryQueue.dequeue()
else:
return None  # Or appropriate error/indicator

# Method to get the top element of the stack
def top():
# Return the front element of primaryQueue without dequeuing it
if not primaryQueue.is_empty():
return primaryQueue.front()
else:
return None  # Or appropriate error/indicator

# Method to check if the stack is empty
def is_empty():
# Return whether primaryQueue is empty
return primaryQueue.is_empty()

Step-by-Step Explanation:

Initialization (
initialize

) :

Two empty lists act as queues
primaryQueue

and
secondaryQueue

.

Push (
push

) operation :

The element is first enqueued to
secondaryQueue

to keep it at the front.
All remaining elements in
primaryQueue

are moved to
secondaryQueue

.
This swap ensures the most recently added element is always positioned correctly for LIFO behavior.

Pop (
pop

) operation :

Directly dequeues from
primaryQueue

, which returns the top of the stack due to our rearrangement during
push

.

Top (
top

) operation :

Provides access to the front of
primaryQueue

without removing it, revealing the top stack element.

Empty (
is_empty

) operation :

Simply checks if
primaryQueue

is empty to determine if the stack is empty.

In implementing this, the essence is to understand the rearrangement process during a push to ensure the stack operates correctly. This allows passing all operations through a queue while maintaining stack behavior.