Convert Sorted Array to Binary Search Tree (C++ code)

LeetCode Convert Sorted Array to Binary Search Tree, Oct 2 '12
 G家面试题，写一个吧。
Given an array where elements are sorted in ascending order, convert it to a height balanced BST.

TreeNode *sortedArrayToBST(vector<int> &num, int start, int end){
    if(start > end) return NULL;
    int mid = (start + end)/2;
    TreeNode *root = new TreeNode(num[mid]);
    root->left = sortedArrayToBST(num, start, mid - 1);
    root->right = sortedArrayToBST(num, mid+1, end);

   return root;
}


TreeNode *sortedArrayToBST(vector<int> &num) {
       if(num.size() == 0) return NULL; 

       return sortedArrayToBST(num, 0, num.size() - 1); 
       
    }