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