convert LinkedList into AVL tree

AvlTree.java

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class AvlTree {
    TreeNode root = null;
    public TreeNode sortedListToBST(ListNode head) {
        while(head!=null){
            this.root = insert(head.val, this.root);
            head = head.next;
        }
        return this.root;
    }
    public TreeNode leftRotate(TreeNode t){
         TreeNode newNode = t.right;
        t.right = newNode.left;
        newNode.left = t;
        return newNode;
        
    }
    public TreeNode rightRotate(TreeNode t){
        TreeNode newNode = t.left;
        t.left = newNode.right;
        newNode.right = t;
        return newNode;
    }
    public TreeNode insert (int val, TreeNode root){
        if(root ==null){
            return new TreeNode(val);
        } 
        else if( root.val > val){
            root.left =  insert(val, root.left);
        } else {
            root.right = insert(val,root.right);
        } 
        
        // check if balance is greater than 2 
        if(balance(root) > 1) {
            if(balance(root.left) < 0 )
                root.left = leftRotate(root.left);
            root = rightRotate(root);
        } else if( balance (root) < 1 ){
            if(balance(root.right) > 0) 
                root.right = rightRotate(root.right);
            root = leftRotate(root);
        }
        //displayNode(root);
        return root;
        
    }
    //get balance of tree 
    public int balance(TreeNode t){
        return height(t.left) - height(t.right);
    }
    // calculate height of the tree
    public int height(TreeNode t){
        if(t==null)
            return 0;
        else
            return 1+ max(height(t.left), height(t.right));
    }
    public int max( int a ,int b){
        return  (a > b) ? a : b;
    }
    public void displayNode(TreeNode root){
        if(root == null)
            return;
        System.out.print(" "+root.val);
        displayNode(root.left);
        displayNode(root.right);
        // System.out.print(""+root.val+ "[");
        // displayNode(root.left);   
        // displayNode(root.right);      
        // System.out.print("]");
    }
}