The 23rd International Olympiad in Informatics was held in Pattaya, Thailand in 2011. There were two competition days, with 3 tasks given to the competitors on each day. You can see the tasks, test cases and solutions below.

Day 1:
      Tropical Garden
      Race
      Ricehub

Day 2:
      Crocodile
      Elephants
      Parrots

Other materials:
      Test cases
      Solutions to all problems


Note that the grading model changed at IOI'94 and beyond.
Before IOI 1994, a human evaluator would sit behind the competitor's computer and do the testing manually (enter test data through the keyboard, check the output on the screen). Starting from IOI 1994, an automatic grading process is used, with test input defined in files.