Add solution of problem 1
Add solution of problem 2
Add solution of problem 3
Add solution of problem 4
Add solution of problem 5
Add solution of problem 6
Add solution of problem 7
Add solution of problem 8
Add solution of problem 9
Add solution of problem 10
Ignore .DS_Store
Add solution of problem 11
Add solution of problem 12
Add solution of problem 13
Add solution of problem 14
Add solution of problem 15