Adding .DS_Store to .gitignore.
Adding solution for problem 1.
Adding solution for problem 2.
Adding solution for problem 3.
Adding solution for problem 4.
Adding solution for problem 5.
Adding solution for problem 6.
Adding solution for problem 7.
Adding solution for problem 8.
Adding solution for problem 9.
Adding solution for problem 10.
Adding solution for problem 11.
Adding solution for problem 12.
Adding solution for problem 13.
Adding solution for problem 14.
Adding solution for problem 15.
Adding solution for problem 16.
Adding solution for problem 17.
Adding solution for problem 18.
Adding solution for problem19.