I see that dijkstra (and some shortest path algorithm) have a lot of variants like the roads have color, money and time to pass on a road,... etc. So, how to I practice them and how should I thinking for these problem.

Anyone please help me, I'm very need a help now, my national team selection contest is coming.

I'm currently learning fenwick tree and I find it quite difficult (in my opinion, it's more difficult than segment tree). As far as I know, most problems that can be solved with a fenwick tree can also be solved with a segment tree. So, in addition to memory optimization and simple usage, is there anything more optimal about fenwick tree than segment tree? And, sure, can anyone give me websites to use to learn fenwick tree? Thank you.