Please help to solve this problem on spoj, can’t understand how to use binary search.
Check out this link
Simple binary search is a technique which simply finds whether an element is present in a (sorted) array in log(N) time.
However, it can be tweaked in order to perform a world of computations, usually involving the minimum/maximum value of some entity.
It is best explained in the following link :
Take your time, and understand the content of the blog.
Similar problems for practice :
SPOJ - EKO
Codeforces - 782B
Simple and clear explanation http://sahilmutneja.com/blog/2015/02/aggressive-cowsaggrcow-spoj/