Populate sibling pointer in each node for level order traversal

void connect(Node parent)
{
    if (parent == null)
        return;

    // Sibling for the right child will lie in children
    // of current parent's sibling
    // We need to find this distant cousin, even if current
    // parent's right child is null

    Node siblingOfRightChild = null;
    Node uncle = parent.sibling;
    while (uncle != null)
    {
        if (uncle.left != null) // Give preference to left child
        {
            siblingOfRightChild = uncle.left;
            break;
        }
        if (uncle.right != null) // Consider right child, if left is null
        {
            siblingOfRightChild = uncle.right;
            break;
        }
        // Uncle does not have any children, move to next uncle
        uncle = uncle.sibling;
    }
    
    if (parent.left != null)
    {
        if (parent.right != null)
            parent.left.sibling = parent.right;
        else
            parent.left.sibling = siblingOfRightChild;
    }

    if (parent.right != null)
        parent.right.sibling = siblingOfRightChild;

    connect(parent.left);
    connect(parent.right);
}