MyQueue.java

// Time Complexity :  O(1) average
// Space Complexity :O(n)
// Did this code successfully run on Leetcode : yes 
// Any problem you faced while coding this : No

class MyQueue {

    Stack<Integer> in;
    Stack<Integer> out;
    public MyQueue() {
        in = new Stack<>();
        out = new Stack<>();
    }
    
    public void push(int x) {
        in.push(x);
    }
    
    public int pop() {
        if(out.isEmpty()){
            while(!in.isEmpty()){
               out.push(in.pop());
            }
        }
        return out.pop();
    }
    
    public int peek() {
        if(out.isEmpty()){
            while(!in.isEmpty()){
                out.push(in.pop());
            }
        }

        return out.peek();
    }
    
    public boolean empty() {
        
        if(in.isEmpty() && out.isEmpty()){
            return true;
        }

        return false;
    }
}

/**
 * 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();
 */