232. Implement Queue using Stacks

Implement Queue using Stacks

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<Integer> stack1 = new Stack<>();
    Stack<Integer> stack2 = new Stack<>();

    // Push element x to the back of queue.
    public void push(int x) {
        stack1.push(x);
    }

    // Removes the element from in front of queue.
    public void pop() {
           if (!stack2.isEmpty()) {
         stack2.pop();
         return;
        }

        while (!stack1.isEmpty()) {
            stack2.push(stack1.pop());
        }

        stack2.pop();
    }

    // Get the front element.
    public int peek() {
        if (!stack2.isEmpty()) {
            return stack2.peek();
        }

        while (!stack1.isEmpty()) {
            stack2.push(stack1.pop());
        }

        return stack2.peek();
    }

    // Return whether the queue is empty.
    public boolean empty() {
        return stack2.isEmpty() && stack1.isEmpty();
    }
}