Single Right Rotation

Consider inserting the values $7, 5, 3$ in that order.

We start by inserting $7$:

We have a BST with a single node; it is balanced!

Next, we insert $5$.

Now our BST has two nodes. Notice the height and balance factor of the root has changed (and it is still balanced).

Next, we insert $3$.

Our BST has three nodes now. Notice the heights and balance factors of the parent and grandparent of $3$ have changed. In particular, the grandparent (the root) is not balanced anymore!

However, if we were to push $7$ to the right of $5$, we would restore balance:

This is called a (single) right rotation:

Notice the violation of balance property occurred in the grandparent of the newly inserted node. From the perspective of the grandparent node, the problem was in the left child's left subtree. The solution is a (single) right rotation to bring the parent node (median value) above the grandparent (high value).