《leetCode》:Implement Stack using Queues

タイトル

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).

構想
この問題は比較的に簡単で、2つのキューを借りてスタックを実現すればいい.キューは:先進先出、スタックは:先進後出であることを理解しなければならない.この二つの点が分かれば実現しやすい.

class MyStack {
    Queue<Integer> q1=new LinkedList<Integer>();
    Queue<Integer> q2=new LinkedList<Integer>();
    // Push element x onto stack.
    public void push(int x) {
        q1.add(x);
    }

    // Removes the element on top of the stack.
    public void pop() {
        if(q1.isEmpty()){
            return;
        }
        else{
            int len=q1.size();
            for(int i=0;i<len-1;i++){
                q2.add(q1.poll());
            }
            q1.poll();//        。
            for(int i=0;i<len-1;i++){
                q1.add(q2.poll());
            }
        }
    }

    // Get the top element.
    public int top() {
        if(q1.isEmpty()){
            return -1;
        }
        else{
            int len=q1.size();
            for(int i=0;i<len-1;i++){
                q2.add(q1.poll());
            }
            int res=q1.poll();//        。
            q2.add(res);
            for(int i=0;i<len;i++){
                q1.add(q2.poll());
            }
            return res;
        }
    }

    // Return whether the stack is empty.
    public boolean empty() {

        return q1.isEmpty();
    }
}

小結
寝室に帰る前に、leetcodeでこの2つの簡単な問題をACして、やはりとても楽しかったです.