https://class.coursera.org/algorithms-001/lecture
http://open.163.com/special/opencourse/algorithms.html
http://www.comp.nus.edu.sg/~stevenha/programming/acmoj.html#Q7
http://www.algorithmist.com/index.php/Main_Page
http://acm.timus.ru/problem.aspx?space=1&num=1306
http://hzwer.com/1234.html
http://xsy.gdgzez.com.cn/JudgeOnline/