LC225,232 Implement Stack/Queue via the other

225. Implement Stack using Queues:

关键: Queue中除最后一个元素前面的元素整体搬迁到Queue后面, 这样最后一个元素就到最前面了。

class MyStack {

    Queue<Integer> q;
    /** Initialize your data structure here. */
    public MyStack() {
        q = new LinkedList<>();
    }
    
    /** Push element x onto stack. */
    public void push(int x) {
        q.offer(x);
        for (int i = 1; i < q.size(); ++i) {
            q.offer(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();
    }
}

/**
 * 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();
 * boolean param_4 = obj.empty();
 */

---

232. Implement Queue using Stacks:

关键: 入，出分别是两个stack，相反的操作把LIFO变为了FIFO.

class MyQueue {

    Stack<Integer> s1;
    Stack<Integer> s2;
    /** Initialize your data structure here. */
    public MyQueue() {
        s1 = new Stack<>();
        s2 = new Stack<>();
    }
    
    /** Push element x to the back of queue. */
    public void push(int x) {
        s1.push(x);
    }
    
    /** Removes the element from in front of queue and returns that element. */
    public int pop() {
        peek(); // 作为tmp的s1搬到s2
        return s2.pop();
    }
    
    /** Get the front element. */
    // s2非空，就直接是s2最上面的元素
    // s2是空，那么先把FIFO的s1元素全部倒过来放进s2, 再从s2 pop
    public int peek() {
        if (!s2.empty()) {
            return s2.peek();
        } else {
            while (!s1.empty()) {
                s2.push(s1.pop());
            }
        }
        return s2.peek();
    }
    
    /** Returns whether the queue is empty. */
    public boolean empty() {
        return s1.empty() && s2.empty();
    }
}

/**
 * Your MyQueue object will be instantiated and called as such:
 * MyQueue obj = new MyQueue();
 * obj.push(x);
 * int param_2 = obj.pop();
 * int param_3 = obj.peek();
 * boolean param_4 = obj.empty();
 */