No225.implement-stack-using-queues.cs

/*
 * Difficulty:
 * Easy
 *
 * Desc:
 * 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).
*/

public class MyStack {
    Queue<int> queue;

    /** Initialize your data structure here. */
    public MyStack() {
        queue = new Queue<int>();
    }
    
    /** Push element x onto stack. */
    public void Push(int x) {
        queue.Enqueue(x);
    }
    
    /** Removes the element on top of the stack and returns that element. */
    public int Pop() {
        int count = queue.Count - 1;
        while (count-- > 0) queue.Enqueue(queue.Dequeue());
        return queue.Dequeue();
    }
    
    /** Get the top element. */
    public int Top() {
        int count = queue.Count - 1;
        while (count-- > 0) queue.Enqueue(queue.Dequeue());
        int res = queue.Dequeue();
        queue.Enqueue(res);
        return res;
    }
    
    /** Returns whether the stack is empty. */
    public bool Empty() {
        return queue.Count == 0;
    }
}

/**
 * Your MyStack object will be instantiated and called as such:
 * MyStack obj = new MyStack();
 * obj.Push(x);
 * int param_2 = obj.Pop();
 * int param_3 = obj.Top();
 * bool param_4 = obj.Empty();
 */