I've solved this problem in C++. But I can't get AC in Java. Some clever guy from america told me that there's a thread checking my program and because it is not using tree, it uses only recursive calls this checking thread also uses stack. And in result this special thread causes stack overflow. He suggested me to use "java -Xint p1136". p1136 is my class. It worked on my computer. But I think on the server there's no one used this option. Am I right? Does it mean that we must use binary tree to solve this problem and no other way for recursive solution?

Just look at the last paragraph of java faq:
http://acm.timus.ru/java.aspx

You want me to use maximum 3000 threads?

Oh, I've done it. Without so many threads.:)