What I have done and what I have learnt from comptetition problems:

From Codeforces,I solved the following problems:

1) Files list
    Previously in OS or c program, we can have 8 characters filename and 3 characters for extension.
I felt that design in this problem
   http://codeforces.com/problemset/problem/174/B

2)cd and pwd commands
http://codeforces.com/problemset/problem/158/C
 From this problem, I learnt that how we can implement cd and pwd commands in shell.
How it will remembers the user's present working directory especially the user is doing cd ..

3)BHTML & BCSS
http://codeforces.com/problemset/problem/172/E

  From this problem, I learnt that How css is applied with HTML tags. How we can highlight the tags with user's choices.

4)Simple XML
http://codeforces.com/problemset/problem/125/B

  From this problem I learnt that How XML tags can be aligned or C/C++ source code is aligned based on {} brackets in editor

5)try... catch problem
http://codeforces.com/problemset/problem/195/C

From this problem, I learnt how compilers will execute exceptions with try ... catch blocks
It is one of awesome reallife problem.