title: time complexity of merge sort
body: <p>why the time complexity of best case of top-down merge sort is in O(nlogn)?
  i think the best case of top-down merge sort is 1, only need to compare 1 time.
  how about the time complexity of bottom-up merge sort in worst case, best case and average case.</p>
  
  <p>One more question is  why each iteration takes exactly O(n)? could some one help with that?</p>