I am in 2nd year of CS but in our curriculum we have algorithms in 3rd year and data structures currently.
But I wish to prepare them now as I have much time which I don’t feel would be there in 3rd year.
this is best organized way that you will find. and refer geeks and tushar roy videos for trees and for graphs, follow hackerearth.com , its the best place to learn , n moreover the link has problems for practice for every algorithms, so do not just read and leave , do solve problems to have a more clear grasp on the concepts.
All good resources are mentioned by @raj79 just want to add one more Do Read CLRS also termed as bible of algorithms it is the best resource to learn algorithms.
if u r a beginner in algorithms then dont head towards CLRS start off your journey with geeksforgeeks and slowly you will automatically realize whether you should move towards CLRS or not. CLRS demands good grasp of Discrete maths so choose accordingly