Solutions, explained

How to use these solutions

Each solution breaks the problem into approaches, usually from brute force to the best one, and explains why each works. Code comes in Python, Java, C++ and other languages. Knowing why an approach works matters more in an interview than remembering the code.

• Try the problem first. Give it at least 20 minutes before you open the solution.
• Read every approach. The brute force shows you the problem; the faster ones show the pattern.
• Check the complexity. Note the time and space of each approach and what makes the difference.
• Write it yourself. Close the solution and code it from memory in the editor.
• Come back in a week. Solving it again later is what makes the pattern stick.