42. Trapping Rain Water

https://gist.github.com/cd4159bf4d1b5d31fa27f13062e1681b

322. Coin Change

https://gist.github.com/e383d1c66294ddc044a77e1a8e3ca529

518. Coin Change 2

https://gist.github.com/f4408eb706be4b337494af0dd0e24c47