Implement Queue using Stacks

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.

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, 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).

Solution:

class MyQueue {
    Stack stack = new Stack();
    
    // Push element x to the back of queue.
    public void push(int x) {
        if (stack.empty()) {
            stack.push(x);
            return;
        }
        
        Stack temp = new Stack();
        while (!stack.empty()) {
            temp.push(stack.pop());
        }
        
        stack.push(x);
        while(!temp.empty()) {
            stack.push(temp.pop());
        }
    }

    // Removes the element from in front of queue.
    public void pop() {
        stack.pop();
    }

    // Get the front element.
    public int peek() {
        return stack.peek();
    }

    // Return whether the queue is empty.
    public boolean empty() {
        return stack.empty();
    }
}