Implement Stack using Queues LeetCode Solution – Queslers

Problem – Implement Stack using Queues LeetCode Solution

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?

Implement Stack using Queues LeetCode Solution in C++

class Stack {
    queue<int> q;
public:
    void push(int x) {
        q.push(x);
        for (int i=1; i<q.size(); i++) {
            q.push(q.front());
            q.pop();
        }
    }

    void pop() {
        q.pop();
    }

    int top() {
        return q.front();
    }

    bool empty() {
        return q.empty();
    }
};

Implement Stack using Queues LeetCode Solution in Java

class MyStack {

    private Queue<Integer> queue = new LinkedList<>();

    public void push(int x) {
        queue.add(x);
        for (int i=1; i<queue.size(); i++)
            queue.add(queue.remove());
    }

    public void pop() {
        queue.remove();
    }

    public int top() {
        return queue.peek();
    }

    public boolean empty() {
        return queue.isEmpty();
    }
}

Implement Stack using Queues LeetCode Solution in Python

class Stack:

    def __init__(self):
        self._queue = collections.deque()

    def push(self, x):
        q = self._queue
        q.append(x)
        for _ in range(len(q) - 1):
            q.append(q.popleft())
        
    def pop(self):
        return self._queue.popleft()

    def top(self):
        return self._queue[0]
    
    def empty(self):
        return not len(self._queue)

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