Add solutions to AtCoder problems

[harshita130602]

Make a folder with name AtCoder and add solutions to problems. Please add the problem statement in comment at the top of the solution code.

[varunkmr19]

Is it still open? I would love to contribute.

[harshita130602]

Yes it is.

[varunkmr19]

I've made a pr #410
please review and merge it if you want.