Merge pull request #3 from ping128/master

Solution to Problem 4280
Merge pull request #4 from ping128/master

Solutions to Problem 4213, 4877, 4905, 5945, 6088
Merge pull request #5 from ping128/master

Solutions to Problem 4131, 4794, 2728, 4123. Way to go, Siwakorn!
A few graph problems.
A few more graph problems.
Final batch of graph problems.
First pass at slides