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());    }}