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.
* struct ListNode {
*     int val;
*     ListNode *next;
*     ListNode(int x) : val(x), next(NULL) {}
* };
*/
/**
* Definition for binary tree
* struct TreeNode {
*     int val;
*     TreeNode *left;
*     TreeNode *right;
*     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
* };
*/
class Solution {
public:
TreeNode *vectorToBST(vector<int>&a, int start, int end)
{
if(start>end)
return NULL;
int mid = (start+end)/2;
TreeNode *Thead = new TreeNode(a[mid]);
Thead->left  = vectorToBST(a,start,mid-1);
Thead->right = vectorToBST(a,mid+1,end);
return Thead;
}
TreeNode *sortedListToBST(ListNode *head)
{
if(NULL==head)
return NULL;
if(NULL==head->next)
{
TreeNode *Thead = new TreeNode(head->val);
return Thead;
}
vector<int>valVec;
valVec.clear();
while(head)
{
valVec.push_back(head->val);
head = head->next;
}
return vectorToBST(valVec,0,valVec.size()-1);
}
};

i dont know what's the usage of "sorted List",

unsorted also be ok