Program  complexity 

      = a  NlogN +  bN + C

      = a  NlogN +  O(N)

       ( a  NlogN  =>  leading term)

1	const	each statement constant times
logN	log	(each  step  half   input)
N	liner	(divide and conquer)
NlogN
	quadratic	(recurrent relation)
	cublic
	exponential

Common Mistakes

• Ignore                 performance   : large  N -  efficient program

• Too concerned performance  :   small  N -  simple program