232. Implement Queue using Stacks

Implement the following operations of a queue using stacks.

• push(x) -- Push element x to the back of queue.
• pop() -- Removes the element from in front of queue.
• peek() -- Get the front element.
• empty() -- Return whether the queue is empty.

Example:

MyQueue queue = new MyQueue();

queue.push(1);
queue.push(2);  
queue.peek();  // returns 1
queue.pop();   // returns 1
queue.empty(); // returns false

Notes:

• You must use only standard operations of a stack -- which means only push to top, peek/pop from top, size, and is empty operations are valid.
• Depending on your language, stack may not be supported natively. You may simulate a stack by using a list or deque (double-ended queue), as long as you use only standard operations of a stack.
• You may assume that all operations are valid (for example, no pop or peek operations will be called on an empty queue).

用两个stack，s1只记录新加入的元素，即push永远只push进s1

s2用作pop：当s2非空时，直接pop；当s2为空，先把s1中所有元素pop进s2，再pop s2

push() 　　-- time: O(1), space: O(1)

pop() 　　 -- amortized O(1), space: O(n)

peek()　    -- amortized O(1), space: O(n)

size() 　　 -- time: O(1), space: O(1)

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

    /** Initialize your data structure here. */
    public MyQueue() {
        s1 = new LinkedList<>();
        s2 = new LinkedList<>();
        size = 0;
    }
    
    /** Push element x to the back of queue. */
    public void push(int x) {
        s1.push(x);
        size++;
    }
    
    /** Removes the element from in front of queue and returns that element. */
    public int pop() {
        if(s2.isEmpty()) {
            while(!s1.isEmpty()) {
                s2.push(s1.pop());
            }
        }
        size--;
        return s2.pop();
    }
    
    /** Get the front element. */
    public int peek() {
        if(s2.isEmpty()) {
            while(!s1.isEmpty()) {
                s2.push(s1.pop());
            }
        }
        return s2.peek();
    }
    
    /** Returns whether the queue is empty. */
    public boolean empty() {
        return size == 0;
    }
}

/**
 * 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();
 */