232. Implement Queue using Stacks

```java
class MyQueue {
    Stack<Integer> stack1;
    Stack<Integer> stack2;

    public MyQueue() {
        stack1 = new Stack<>();
        stack2 = new Stack<>();
    }
    
    public void push(int x) {
        popAllToStack1();
        stack1.push(x);
    }
    
    public int pop() {
        popAllToStack2();
        return stack2.pop();
    }
    
    public int peek() {
        popAllToStack2();
        return stack2.peek();
    }
    
    public boolean empty() {
        return stack1.isEmpty() && stack2.isEmpty();
    }

    private void popAllToStack2() {
        while (!stack1.isEmpty()) {
            stack2.push(stack1.pop());
        }
    }

    private void popAllToStack1() {
        while (!stack2.isEmpty()) {
            stack1.push(stack2.pop());
        }
    }
}

/**
 * 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();


2 1
1 2

if we want push -> push second back

keep pushing

2
1
when peek, pop
pop all to stack2
    1
    2
if push again, pop all from stack 2 to 1

3
2
1


empty, check both stacks

T: O(xN)
 */
```

better idea

only when stack2 is Empty , we need to push stack1 to stack2, why?

because we already have queue pop and peak candidate, just pop or peek it from stack2

```java
class MyQueue {
    Stack<Integer> stack1;
    Stack<Integer> stack2;

    public MyQueue() {
        stack1 = new Stack<>();
        stack2 = new Stack<>();
    }
    
    public void push(int x) {
        stack1.push(x);
    }
    
    public int pop() {
        popAllToStack2();
        return stack2.pop();
    }
    
    public int peek() {
        popAllToStack2();
        return stack2.peek();
    }
    
    public boolean empty() {
        return stack1.isEmpty() && stack2.isEmpty();
    }

    private void popAllToStack2() {
        if (stack2.isEmpty()) {
            while (!stack1.isEmpty()) {
                stack2.push(stack1.pop());
            }
        }
    }
}

/**
 * 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();

 */
```