C Program for Red Black Tree Insertion

Following article is extension of article discussed here.
In AVL tree insertion, we used rotation as a tool to do balancing after insertion caused imbalance. In Red-Black tree, we use two tools to do balancing.

1. Recoloring 
2. Rotation

We try recoloring first, if recoloring doesn't work, then we go for rotation. Following is detailed algorithm. The algorithms has mainly two cases depending upon the color of uncle. If uncle is red, we do recoloring. If uncle is black, we do rotations and/or recoloring.
Color of a NULL node is considered as BLACK.

Let x be the newly inserted node. 

1. Perform standard BST insertion and make the color of newly inserted nodes as RED.
2. If x is root, change color of x as BLACK (Black height of complete tree increases by 1).
3. Do following if color of x's parent is not BLACK or x is not root. 
   1. If x's uncle is RED (Grand parent must have been black from property 4) 
      1. Change color of parent and uncle as BLACK. 
      2. color of grand parent as RED. 
      3.     3. Change x = x's grandparent, repeat steps 2 and 3 for new x. 
          

👁 redBlackCase2

      2. If x's uncle is BLACK, then there can be four configurations for x, x's parent (p) and x's grandparent (g) (This is similar to AVL Tree) 

1. Determine the configuration:
   1. Left Case (p is left child of g and x is left child of p).
   2. Left Right Case (p is left child of g and x is right child of p).
   3. Right Right Case (Mirror of case a).
   4. Right Left Case (Mirror of case c).
2. Change x = x’s parent, repeat steps 2 and 3 for new x.

Following are operations to be performed in four subcases when uncle is BLACK.

All four cases when Uncle is BLACK

Left Left Case (See g, p and x) 

👁 redBlackCase3a

Left Right Case (See g, p and x) 

👁 redBlackCase3b

Right Right Case (See g, p and x) 

👁 redBlackCase3c

Right Left Case (See g, p and x) 

👁 redBlackCase3d

Examples of Insertion 

👁 Examples

Implementation:

Inorder Traversal of Created Tree
1 2 3 4 5 6 7 

Level Order Traversal of Created Tree
6 4 7 2 5 1 3 

Time Complexity:  O(log n), as the height of red-black tree is O(log n) at most, and complexity of rotation is constant.
Auxiliary Space:  O(n),  here 'n' is the number of nodes in the red-black trees.