排序列表转换为二分查找树

public class Solution {
/**weizuo
* @param head: The first node of linked list.
* @return: a tree node
* 排序列表转换为二分查找树
* 给出一个所有元素以升序排序的单链表，将它转换成一棵高度平衡的二分查找树
*/
private ListNode current;

private int getListLength(ListNode head) {
int size = 0;

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

return size;
}

public TreeNode sortedListToBST(ListNode head) {
int size;

current = head;
size = getListLength(head);

return sortedListToBSTHelper(size);
}

public TreeNode sortedListToBSTHelper(int size) {
if (size <= 0) {
return null;
}

TreeNode left = sortedListToBSTHelper(size / 2);
TreeNode root = new TreeNode(current.val);
current = current.next;
TreeNode right = sortedListToBSTHelper(size - 1 - size / 2);

root.left = left;
root.right = right;

return root;
}
}