The purpose of LeetCode is to provide you hands-on training on real coding interview questions. The Online Judge gives you immediate feedback on the correctness and efficiency of your algorithm which facilitates a great learning experience.
The mission of LeetCode is to help software engineers enhance their skills, expand their knowledge, and prepare for technical interviews. They have over 1800 questions broken up into 3 skill levels: easy, medium, and hard. In addition, Premium members get access to problem sets based on company questions.