Load Balancer and EC2 in same subnet?
The best practice is to have your web, app and db servers in private subnets and only the actual front end - which is ALB in your case - in the public subnet. The reason is that web, app and db servers shouldn't be accessible from outside even if by accident (e.g. through security group left too open).

Technically it will still work to have both the ALB and the instances in public subnet but it's not recommended for the above reason. Also make sure to lock down access to the instance port 80 and 443 only from the ALB SG if you don't want any direct traffic.

Hope that helps :)