MyQueue.java

import java.util.*;

class MyQueue {

    Stack<Integer> inStack;
    Stack<Integer> outStack;

    public MyQueue() {

        this.inStack = new Stack<>();
        this.outStack = new Stack<>();

    }

    public void push(int x) {
        inStack.push(x);
    }

    public int pop() {

        if (outStack.isEmpty() && inStack.isEmpty()) {
            return -1;
        }

        peek();
        return outStack.pop();

    }

    public int peek() {

        if (outStack.isEmpty()) {
            while (!inStack.isEmpty()) {
                outStack.push(inStack.pop());
            }
        }
        return outStack.peek();
    }

    public boolean empty() {

        return inStack.isEmpty() && outStack.isEmpty();

    }
}

// TC = O(n)
// SC = O(n)

/**
 * 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();
 */