題目

注意棧為空的判斷，如果是空的，要返回-1

class CQueue {
    Stack<Integer> stack1 ;
    Stack<Integer> stack2;
 
    public CQueue() {
        stack1 = new Stack<>();
        stack2 = new Stack<>();
    }
    
    public void appendTail(int value) {
        stack1.push(value);
    }
    
    public int deleteHead() {
        if (stack2.isEmpty()) {
            while (!stack1.isEmpty()) {
                stack2.push(stack1.pop());
            }
           
        }
        return stack2.isEmpty() ? -1 : stack2.pop();
    }
}