232 Implement Queue using Stacks

class MyQueue {
// queue :  5  4  3  2 1 ->
// stack :  1. 2. 3. 4 5 ->
    // push element 
    //  stack1    1 2 3->
    //  stack2    3 2       1 ->
    /** Initialize your data structure here. */

    Stack<Integer> stack1;
    Stack<Integer> stack2;

    public MyQueue() {
        stack1 = new Stack<>();
        stack2 = new Stack<>();
    }

    /** Push element x to the back of queue. */
    public void push(int x) {
        stack1.push(x);
    }

    /** Removes the element from in front of queue and returns that element. */
    public int pop() {
        if (stack2.isEmpty()) {
            while (!stack1.isEmpty()) {
                stack2.push(stack1.pop());
            }
        }
        return stack2.pop();
    }

    /** Get the front element. */
    public int peek() {
        if (stack2.isEmpty()) {
            while (!stack1.isEmpty()) {
                stack2.push(stack1.pop());
            }
        }
        return stack2.peek();
    }

    /** Returns whether the queue is empty. */
    public boolean empty() {
        return stack1.isEmpty() && stack2.isEmpty();
    }
}

/**
 * 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();
 */

class MyQueue {
     Stack<Integer> s1;
    Stack<Integer> s2;
    private int front;
    /** Initialize your data structure here. */
    public MyQueue() {
        s1 = new Stack<>();
        s2 = new Stack<>();
    }

    // time : O(n);
    /** Push element x to the back of queue. */
    public void push(int x) {
        while (!s1.isEmpty()) {
            s2.push(s1.pop());
        }
        s2.push(x);
        while (!s2.isEmpty()) {
            s1.push(s2.pop());
        }
    }

    /** Removes the element from in front of queue and returns that element. */
    public int pop() {
        return s1.pop();
    }

    /** Get the front element. */
    public int peek() {
        return s1.peek();
    }

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

}

/**
 * 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();
 */