Princeton's online course on Coursera's platform is the best:
Algorithms by Robert Sedgewick and Kevin Wayne.
Algorithms, Part I -- by Robert Sedgewick and Kevin Wayne.
https://www.coursera.org/learn/algorithms-part1
Algorithms, Part II
https://www.coursera.org/learn/algorithms-part2
The corresponding textbook is:
Algorithms, 4th Edition by Robert Sedgewick and Kevin Wayne.
Its site:
https://algs4.cs.princeton.edu/home/
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After that, you could also study & use the classic CLRS book:
Introduction to Algorithms, by Thomas H. Cormen, Charles E. Leiserson, Ronald L. Rivest, and Clifford Stein.