Amazon Software Engineer interview questions

something related to virtual function. not most difficult question as such but i did not know the answer

It was an extended question regarding to binary tree. The tree consists of the numbers and the mathematics operations, like +, -, *, /. The root is the operator and its two children is the numbers. The question is how to calculate the result of the whole tree. I suggested to DFS the tree to reach the bottom. But my interviewer wanted me to use another way to solve the problem. I finally use recursion to solve the problem but it consumed too much time and I even didn't have time to do the second question.