I appered for Hackerrank intermediate problem solving certification .
came accross the below mentioned problem and was stuck  for more then an hour . 

Let there are n number of students and every students has predefinded 
skillLevel ,S where S[i] defines skillLevel of ith student .what is the 
maximum number of pairs that can be formed such that the difference 
between the skillLevel of students in a pair is not less then a number 
,minDiff.

Constraints :
1<n<=2*10^5
0<=S[i]<=10^9
0<=minDiff<=10^5