225. Implement Stack using Queues (Easy)

https://leetcode.com/problems/implement-stack-using-queues/

Implement the following operations of a stack using queues.

• push(x) -- Push element x onto stack.
• pop() -- Removes the element on top of the stack.
• top() -- Get the top element.
• empty() -- Return whether the stack is empty.

Example:

MyStack stack = new MyStack();

stack.push(1);
stack.push(2);  
stack.top();   // returns 2
stack.pop();   // returns 2
stack.empty(); // returns false

Notes:

• You must use only standard operations of a queue -- which means only push to back, peek/pop from front, size, and is empty operations are valid.
• Depending on your language, queue may not be supported natively. You may simulate a queue by using a list or deque (double-ended queue), as long as you use only standard operations of a queue.
• You may assume that all operations are valid (for example, no pop or top operations will be called on an empty stack).

Solutions

class MyStack {

    int size = 0;
    List<Integer> queue = new ArrayList<>();

    /** Initialize your data structure here. */
    public MyStack() {

    }

    /** Push element x onto stack. */
    public void push(int x) {
        size++;
        queue.add(x);
    }

    /** Removes the element on top of the stack and returns that element. */
    public int pop() {
        size--;
        return queue.remove(size);
    }

    /** Get the top element. */
    public int top() {
        return queue.get(size - 1);
    }

    /** Returns whether the stack is empty. */
    public boolean empty() {
        return size == 0;
    }
}

Incorrect Solutions

References