Thanks to @JohnDrolte
for helping to put this together.
If your field of study is computer science at Harvard Extension School, but you'd like to know what courses to take that would most closely follow a Harvard College computer science student, here's the pathway to follow. This pathway is very algorithm-heavy.
CS51 requires CS20 which requires at least Math15
CS61 requires CS26* which requires CS22*
CS121 requires CS20 and CS22 with grades of B+ or higher.
CS124 requires CS22 (be warned, 124 is known to be exceedingly difficult)
*Most Harvard College students typically don't take CS26 or CS22.
Alternatively, a less difficult but still challenging approach to gain a solid foundation in computer science would be to take the following path (note that Harvard College students typically don't take these classes with the exception of CS20):
CSCI E-10B (requires 10a)
CSCI E-22 (requires 10b)
CSCI E-66 (requires 22)
*I found E-20 to be somewhat helpful for E-22, and it's only of the only math courses that is required, so you may want to take it before E-20.
Any further suggestions or advice on what the best CSCI path is are welcome!