Google automatically scans web searches to check for people they'd like to recruit. If you search their kind of thing, they'll offer you a series of coding challenges. Pass the first two, and you can invite a friend. Pass three, and you can send Google your CV. Pass 4 and 5, then you can invite yet more friends. Level 3 consists of 3 ~ medium-leetcode-difficulty problems. Levels 4 is substantially harder, and you're given weeks to complete the single problem in Level 5.
You are, of course, allowed to reference algorithms when constructing you're code, but that's substantially less fun. I am storing my solutions, which I generated myself, here. I've also included a couple of implementations I didn't purely generate myself in cases where my own code was too verbose. Someday I'll come back to these implementations and learn from them. For now, they lie ignored.