What is the best website to learn data structures and algorithms for free?

To practice DSA questions I would suggest these website to practice questions and to get better in Problem solving skills.

For beginners, i would suggest to go for HackerRank, HackerEarth and Geeks for Geeks (GFG).

If you are in intermediate level in DSA then I would suggest you to practice DSA questions in Leetcode. And you can give contest in Leetcode as well along with practice. And leetcode is best for Placement / interview questions.

If you have great grip in DSA and you want some challenging and advanced level of Questions to practice in DSA and you want to do Competitive Programming then I would suggest to to go for Codechef and Codeforces and give contest there.

So, you can choose accordingly to your need.