Some solutions for problems on various Online Judges:
(1) CF ==> Codeforces: http://codeforces.com
(2) UVa ==> UVa Online Judge: http://uva.onlinejudge.org
(3) SPOJ ==> Sphere Online Judge: http://www.spoj.com
(4) CodeChef ==> CodeChef: http://www.codechef.com