title: why isn't merge sort O(N^2)? please look at this example
body: <p>A bubble sort of 16 elements takes 15 steps of 2-2 comparison to sort them one by one.</p>
  
  <p>A merge sort firstly divide them into 16 elements, and merge 2 each time, but the overall steps is still 15 if you draw the picture, i.e. 8+4+2+1=15
  in such case, merge cost O(n) while 2-2 comparison cost O(n) so should be O(n^2).
  why is it false?</p>