Question

Given an AVL tree below:

        23
       /    \     
   19        35
  /  \      /  \     
 8   20   27    40
               /
             38
             /
            36

Is it ok to just do a single rotation at 40, to the right? Making it something like this:

        23
      /    \     
   19        35
  /  \      /  \     
 8   20   27    38
               /  \
             36    40

It still conforms tot he AVL property of having -+1 height compared to the left subtree.

In the answer it does a double rotation so the subtree at 35 above would look like this after:

        23
      /    \     
   19        38
  /  \      /  \     
 8   20   35    40
         /  \
        27  36    

I don’t understand when to do a double rotation and when to do a single rotation if they both do not violate the height property.

Answer 1

The double rotation may be due to a specific AVL algorithm in use. Both answers look like valid AVL trees to me.