LeetCode

From LeetCode

It's a website where people–mostly software engineers–practice their coding skills. There are 800+ questions (and growing), each with multiple solutions. Questions are ranked by level of difficulty: easy, medium, and hard. 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.