leetcode--Convert Sorted List to Binary Search Tree

Given a singly linked list where elements are sorted in ascending order, convert it to a height balanced BST.

/**
* 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; }
* }
*/
public class Solution {
public TreeNode sortedListToBST(ListNode head) {
ListNode t = head;
int count = 0;
while(t!=null){
count++;
t = t.next;
}
int[] nums = new int[count];
t = head;
int i=0;
while(t!=null){
nums[i++] = t.val;
t = t.next;
}
return helper(nums, 0, nums.length-1);
}

public TreeNode helper(int[] nums,int start,int end){
if(start>end) return null;
if(start==end) return new TreeNode(nums[start]);
int mid = start+(end-start)/2;
TreeNode t = new TreeNode(nums[mid]);
t.left = helper(nums, start, mid-1);
t.right = helper(nums, mid+1, end);
return t;
}
}