Answers to one of Google's interview questions about weighing 8 balls

 
One of Google's interview questions: there are eight balls of the same size, 7 of which are of the same weight, with a slightly heavier one. How can I use the balance to determine which ball is heavier only twice?

Answer:
For the first time, I took out 6 of them and placed them on the balance, each weighing 3. I can determine where the heavier one is. There are three situations: 3 on the left of the balance, in the 3 on the right of the balance, or in the remaining 2.

For the second time, we can find a very important one. For the first two cases, we just need to take out two of the three, and place them on the balance to know: if they are different, if the two values are the same, the remaining values are the same. In the third case, it will be called by Tianping.