Ethernaut Solutions
If you are interested in Solidity and smart contract development and you still haven't heard about the Ethernaut CTF (capture-the-flag) check it out here. It's an awesome CTF that will help you understand Solidity and major security flaws that have happened in the past.
Try to solve the challenges by yourself and check the solutions only when you have no clue what to do next. Even then I encourage you to read through the Solidity documentation and try to assume where can security flaws hide.
If you are really desperate to look at the solutions, don't just copy/paste them, try to recreate the solution and try to optimize it or modify it so that you can truly understand what it does. Purpose of these CTFs is to teach you stuff that you won't find in basic examples, some of these challenges will require that you go through the Solidity documentation and search for little crevices that you can use to solve these challenges.