Implement Stack by Two Queues 494

Question

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 may assume that all operations are valid (for example, no pop or top operations will be called on an empty stack).

• 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 -- which means only push to back, pop from front, size, and is empty operations are valid.

Solution

水题。同Implement queue by two stacks。

代码如下：

class Stack{
    private Queue<Integer> queue1;
    private Queue<Integer> queue2;

    public Stack(){
        queue1 = new LinkedList<Integer>();
        queue2 = new LinkedList<Integer>();
    }

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

    public int pop(){
        while(queue1.size() != 1){
            queue2.offer(queue1.poll());
        }
        int res = queue1.poll();
        while(!queue2.isEmpty()){
            queue1.offer(queue2.poll());
        }
        return res;
    }

    public int top(){
        while(queue1.size() != 1){
            queue2.offer(queue1.poll());
        }
        int res = queue1.poll();
        queue2.offer(res);
        while(!queue2.isEmpty()){
            queue1.offer(queue2.poll());
        }
        return res;
    }

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