原题链接: https://oj.leetcode.com/problems/convert-sorted-list-to-binary-search-tree/

public TreeNode sortedListToBST(ListNode head) {
    if (head == null) return null;

    ListNode cur = head;
    int size = 0;

    while (cur != null) {
        size++;
        cur = cur.next;
    }

    List<ListNode> list = new ArrayList<ListNode>();
    list.add(head);
    TreeNode tree = sortedListToBSTHelper(list, 0, size - 1);

    return tree;
}

public TreeNode sortedListToBSTHelper(List<ListNode> list, int low, int high) {
    if (low > high) return null;

    int mid = (low + high) / 2;

    TreeNode left = sortedListToBSTHelper(list, low, mid - 1);

    TreeNode root = new TreeNode(list.get(0).val);
    root.left = left;

    list.set(0, list.get(0).next);
    TreeNode right = sortedListToBSTHelper(list, mid + 1, high);
    root.right = right;

    return root;
}