225. 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.

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).

class MyStack {
Queue<Integer> queue1 = new LinkedList<Integer>();
Queue<Integer> queue2 = new LinkedList<Integer>();
Queue<Integer> q = new LinkedList<Integer>();

public void push(int x) {
queue1.offer(x);
}

public void pop() {
while(queue1.size()>1) queue2.offer(queue1.poll());
queue1.poll();
Queue<Integer> q = queue1;
queue1 = queue2;
queue2 = q;
}

public int top() {
while(queue1.size()>1) queue2.offer(queue1.poll());
int x = queue1.poll();
queue2.offer(x);
Queue<Integer> q = queue1;
queue1 = queue2;
queue2 = q;
return x;
}

public boolean empty() {
return queue1.isEmpty();
}
}