   O(N log N)
  I was looking into big O notation. What does O(N log N) mean? Does it mean N * log2 N? Or does it mean that the algorithm takes N for building and log N for searching?   
  It usually means the algorithm's worst case running time is O(nlog2n).You can't tell which part is searching or building from a big O notation.Recurrence equations give more hints at that.O(nlogn) means the algorithm can be bounded by a function like c*nlog2n when n goes to infinity.   ;  It's about algorithms' complexity.   ;  Also note that in big O notation it doesn't matter what base the log is in; they  all only differ by a constant factor.   