leetcode - Convert Sorted Array to Binary Search Tree

/**
* Definition for binary tree
* struct TreeNode {
*     int val;
*     TreeNode *left;
*     TreeNode *right;
*     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
* };
*/
struct TreeNode
{
int val;
TreeNode *left;
TreeNode *right;
TreeNode(int x) : val(x), left(NULL), right(NULL) {}
};
class Solution {
public:
TreeNode *sortedArrayToBST(std::vector<int> &num) {
return ArrayToBST(num,0,num.size()-1);
}
private:
TreeNode *ArrayToBST(std::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 = ArrayToBST(num,start,mid - 1);
root->right = ArrayToBST(num,mid + 1, end);
return root;
}
};