Berkeley specifically has an introductory course, CS 10, that students can take before starting in the CS major sequence with CS 61A, if they do not feel that they can handle CS 61A from the start.
CS 10: http://cs10.org/sp15/
CS 61A: http://www-inst.eecs.berkeley.edu/~cs61a/fa14/about.html
Other schools may also have different introductory sequences or points in the introductory sequence that students can choose.