225. Implement Stack using Queues

Link

Solution

把queue的head元素 重新放到 queue的tail，進行(len-1)次，就可以得到stack的次序

class MyStack {

    /** Initialize your data structure here. */
    Queue<Integer> q;
    public MyStack() {
        q = new LinkedList<>();
    }
    
    /** Push element x onto stack. */
    public void push(int x) {
        q.add(x);
        for(int i = 1; i < q.size(); i++){
            q.add(q.poll());
        }
    }
    
    /** Removes the element on top of the stack and returns that element. */
    public int pop() {
        return q.poll();
    }
    
    /** Get the top element. */
    public int top() {
        return q.peek();
    }
    
    /** Returns whether the stack is empty. */
    public boolean empty() {
        return q.isEmpty();
    }
}