• A simple introduction to Big O Notation:

         https://www.interviewcake.com/article/java/big-o-notation-time-and-space-complexity

• A video that explains Big O Notation:

         https://www.youtube.com/watch?v=V6mKVRU1evU&index=9&list=PLGLfVvz_LVvReUrWr94U-ZMgjYTQ538nT