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LeetCode is an online coding platform offering a vast collection of algorithm and data-structure problems, organized by difficulty and topic, with an interactive online judge, contests, and tutorials to help you practice and prepare for interviews.
More resources on Algorithms Basics
Algorithms, Part I
This course covers the essential information that every serious programmer needs to know about algorithms and data structures, with emphasis on applications and scientific performance analysis of Java implementations. Part I covers elementary data structures, sorting, and searching algorithms. Part II focuses on graph- and string-processing algorithms. All the features of this course are available for free. People who are interested in digging deeper into the content may wish to obtain the textbook Algorithms, Fourth Edition (upon which the course is based) or visit the website algs4.cs.princeton.edu for a wealth of additional material. This course does not offer a certificate upon completion.
Khan Academy Algorithms
Introductory free lessons
Princeton Algorithms 4
Companion site to Sedgewick book, free resources
GeeksforGeeks - Algorithms
Free tutorials and practice for beginners
Visualgo
Interactive visualizations for algorithms basics
freeCodeCamp - Data Structures and Algorithms in JavaScript
Full course for beginners, top rec in recent threads