Since discrete mathematics and competitive programming go hand in hand here are some useful resources available.
[Skienna][1] (Stony Brook University)
[Discrete Mathematics][2] (ADUni.org)
Add to it if you find more useful resources. Thank you and Happy Programming. 
[1]: Skiena's Lectures
[2]: http://www.aduni.org/courses/discrete/index.php?view=cw