leetcode 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.

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).

class MyQueue {
    private List<Integer> stack = new ArrayList<Integer>();
    private List<Integer> tmp = new ArrayList<Integer>();

    public void push(int x) {
       stack.add(x);
    }
    public void pop() {
        int len = stack.size();
        tmp.clear();

        for (int i = len-1; i >= 0; i--) {
            tmp.add(stack.get(i));
        }

        stack.clear();

        for (int i = tmp.size()-2; i >= 0; i--) {
            stack.add(tmp.get(i));
        }
    }
    public int peek() {
        int len = stack.size(), ans = -1;

        for (int i = len-1; i>=0; i--) {
            ans = stack.get(i);
        }
        return ans;
    }
    public boolean empty() {
        return stack.size() == 0;
    }
}
public class Solution232 {
    public static void main(String[] args) {
        MyQueue myQueue = new MyQueue();
        myQueue.push(1);
        myQueue.push(2);
        System.out.println(myQueue.peek());
        myQueue.pop();
        myQueue.pop();
        System.out.println(myQueue.empty());
    }
}