Check if a BST is balanced

IsBTBalanced.cs

    public class TreeNode {
        public int val;
        public TreeNode left;
        public TreeNode right;
        public TreeNode(int x) { val = x; }
    }
    
    public bool IsBalanced(TreeNode root) {
        if(root == null) return true;
        
        return checkHeight(root) != -1;
    }
    
    private int checkHeight(TreeNode root){
        
        if(root == null) return 0;
        
        int lHeight = checkHeight(root.left);
        
        if(lHeight == -1) return -1;
        
        int rHeight = checkHeight(root.right);
        
        if(rHeight == -1) return -1;
        
        if(Math.Abs(rHeight - lHeight) > 1) return -1;
        
        return Math.Max(rHeight, lHeight) + 1;
    }