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Explain single rotations in AVL trees

#1
12-15-2022, 09:24 AM
You see the imbalance hit a node and you twist it back into place with one move. I find that single rotations fix the height difference right away when the heavy side is on the same direction twice. You grab the unbalanced spot and swing its child up while the grandchild slides down to keep order intact. And the tree stays sorted because the left parts stay left and the right parts stay right after the pivot. But the heights update quick so no further checks happen right then.

Now the right rotation comes in when the left child pulls everything too low on that side. I show you how the parent drops to become the right child of its old left kid and the middle subtree moves over without breaking search order. You watch the balance factor flip from negative two back to zero or one in a snap. Perhaps the left side was already heavier and one more insert makes it worse so this single turn saves the day. Then the overall height drops by one and the ancestors feel less strain from that point upward.

Also a left rotation mirrors the same idea but swings the other way when the right child causes the trouble. I tell you the parent becomes the left child of its former right kid while the left subtree of that right kid shifts across. You notice the search property holds because smaller values still sit left of larger ones after the change. Or the height difference corrects itself fast without needing a double turn in simple cases. Maybe you test it on a chain of three nodes and see the middle one rise to the top spot.

The process keeps the binary search rules alive every time you perform the turn. I explain that the new root after rotation has the old parent as its child on the correct side. You check the balance factors on the moved nodes and they settle into the allowed range of minus one to one. But sometimes the rotation affects only that local area and the rest of the tree stays untouched. Then you move up to the parent and see if another imbalance appears higher up the path.

Single rotations handle the straightforward left left or right right patterns without extra steps. I like how they restore the height balance in one action when the pattern matches exactly. You apply the turn by updating three or four pointers at most and the job finishes quick. Perhaps the tree grows on one side only and this fix stops it from leaning too far. And the inorder traversal gives the same sequence of values before and after because nothing gets reordered in value terms.

You keep the subtrees attached properly during the swing so no data gets lost. I notice the old child node takes over as the new top and its former parent drops down naturally. Then the height of the subtree shrinks back to what it should be for that level. But you verify the balance at each affected node to confirm the fix worked all the way. Maybe an extra node on the heavy side caused the two level difference and the rotation evens it out.

The single turn avoids the need for more complex fixes in these matching cases. I walk you through the pointer changes step by step so the structure holds together. You end up with a shorter path from root to the leaves on that branch. Or the parent pointers if tracked get reset to point at the new parent after the move. Perhaps the rotation happens during an insert or delete and you resume checking from the new height.

Heights get recalculated right after the pointers settle into place. I find this keeps the whole tree property strong without extra passes over distant branches. You see the balance factor return to normal and continue searching for other spots if needed. But the single rotation often clears the issue at that level completely. Then the ancestors may need their own checks but the local problem disappears.

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ProfRon
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Explain single rotations in AVL trees - by ProfRon - 12-15-2022, 09:24 AM

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Explain single rotations in AVL trees

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