I got AC! It's nice problem!
My algo works at O(3*N)=O(N).
1)BFS from any vertex and find the most remote vertex (let V);
2)BFS from V and find the most remote vertex (let U);
3)Create path from U and V; vertex(vertexes) in the middle is answer.

Edited by author 04.08.2015 12:56

It can be done easier , look here https://codeforces.com/blog/entry/17974?locale=ru