leetcode 109. Convert Sorted List to Binary Search Tree

class Solution {
    ListNode tmp;
    public TreeNode sortedListToBST(ListNode head) {
        tmp = head;
        ListNode traverse = head;
        int size = 0;
        
        while(traverse != null){
            size++;
            traverse = traverse.next;
        }
        
        return helper(0, size-1);
        
    }
    
    private TreeNode helper(int start, int end){
        if(start > end)
            return null;
        
        int mid = start + (end - start) / 2;
        TreeNode left = helper(start, mid - 1);
        
        TreeNode cur = new TreeNode(tmp.val);
        cur.left = left;
        tmp = tmp.next;
        TreeNode right = helper(mid+1, end);
        cur.right = right;
        
        
        return cur;
    }
}