Sources of problems

• codeforces.com
• leetcode
• 30-Day LeetCoding Challenge

Courses

• Algorithms, Part I (Princeton)
• The Algorithm Design Canvas
• Byte by Byte (Sam Gavis-Hughson)
• Олимпиадные задачи по программированию (Фёдор Меньшиков)

Useful Links

• Coding Interview University - A complete computer science study plan to become a software engineer.
• Tech Interview Handbook - Algorithms study materials, behavioral content and tips for rocking your coding interview
• Cracking the Coding Interview (Gayle Laakmann McDowell)
• Computer Science - by Robert Sedgewick and Kevin Wayne

Credits

Inspired by:

• @aitemr's algo