232. Implement Queue using Stacks

两个栈做法，注意peek和pop方法。考虑两种情况，

1. push(1), push(2), push(3), peek() ：一次性在s1中push1,2,3，再pop(), 要把s1中的元素转移到s2中去。

2. push(1), push(2) ,peek(), push(3), peek()：如果peek转移了，那么s2中情况为3,1,2，第二次调用peek就会得到3，所以调用peek时必须满足s2为空的条件。

根据情况一可以写出while循环，s1->s2转移元素。

while (!s1.isEmpty()) {
    s2.push(elemnts from s1);
}

根据情况二给while循环加上限制条件, s1->s2时，s2必须为空。

if (s2.isEmpty()) {
    while () {...}
}

class MyQueue {
    LinkedList<Integer> s1;
    LinkedList<Integer> s2;

    /** Initialize your data structure here. */
    public MyQueue() {
        this.s1 = new LinkedList<>();
        this.s2 = new LinkedList<>();
    }

    /** Push element x to the back of queue. */
    public void push(int x) {
        this.s1.push(x);
    }

    /** Removes the element from in front of queue and returns that element. */
    public int pop() {
        peek();
        return this.s2.pop();
    }

    /** Get the front element. */
    public int peek() {
        if (this.s2.isEmpty()) {
            while (!this.s1.isEmpty()) {
                this.s2.push(this.s1.pop()); // transfer elements to l2
            }
        }
        return this.s2.peek();
    }

    /** Returns whether the queue is empty. */
    public boolean empty() {
        return s1.isEmpty() && s2.isEmpty();
    }
}