Update:
I recently came to know that, the interviewer was not satisfied the way I coded it. I was going back and forth to change the code without putting an initial plan.

I had a google initial phone screen interview.

The question was to represent a big integer as array of digits. I asked the interviewer for vectors or linked lists. Then opted for vectors. He asked for the basic APIs in the class. Then I was asked to implements a basic mathematical and logical operation. Mathematical operation was implemented successfully. The logic for logical operation was explained but he asked not required to implement as its a simple one.

The interveiwer commented "Good". I was waiting for positive results but after few days got a rejected email. It was really shocking.

When thinking about it I feel while writing the code, I used vector.push_front instead of vector.push_back. This may be the reason for rejection. Prepare well guys. Be strong on basic language constructs and data structures. Hope my experience will help you to prepare better.

Iam planning to try again in some faang companies. I think it require another 12 months waiting period for Google. They say we can try in other posts(different from the one i applied) as soon as i like.