These katas must be solved, they increase in complexity.
- https://www.codewars.com/kata/5556282156230d0e5e000089
- https://www.codewars.com/kata/57a0556c7cb1f31ab3000ad7
- https://www.codewars.com/kata/5556282156230d0e5e000089
- https://www.codewars.com/kata/57a0556c7cb1f31ab3000ad7
- https://www.codewars.com/kata/551f37452ff852b7bd000139
- https://www.codewars.com/kata/57a429e253ba3381850000fb
- https://www.codewars.com/kata/55c45be3b2079eccff00010f
- https://www.codewars.com/kata/57e76bc428d6fbc2d500036d