LeetCode - Convert Sorted List to Binary Search Tree

package bst;

class ListNode {
    int val;
    ListNode next;
    ListNode(int x) { val = x; }
}

public class ConvertSortedListToBinarySearchTree {

    public TreeNode sortedListToBST(ListNode head) {
        int[] nums = convertListToArray(head);
        return constructBST(nums, 0, nums.length - 1);
    }
    
    private TreeNode constructBST(int[] nums, int start, int end) {
        if (start > end) return null;
        int mid = (end - start) / 2 + start;
        TreeNode root = new TreeNode(nums[mid]);
        root.left = constructBST(nums, start, mid - 1);
        root.right = constructBST(nums, mid + 1, end);
        return root;
    }
    
    private int[] convertListToArray(ListNode head) {
        ListNode p = head;
        int count = 0;
        while (head != null) {
            ++count;
            head = head.next;
        }
        
        int[] nums = new int[count];
        count = 0;
        while (p != null) {
            nums[count++] = p.val;
            p = p.next;
        }
        
        return nums;
    }
    
}