Convert Sorted Array to Binary Search Tree

/**
* Definition for a binary tree node.
* struct TreeNode {
*     int val;
*     TreeNode *left;
*     TreeNode *right;
*     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
* };
*/
class Solution {
public:
TreeNode * buildTree(vector<int>& nums,int n,int m)
{
if(n>m)
return NULL;
int middle=(m+n)/2;
TreeNode * left=buildTree(nums,n,middle-1);
TreeNode * right=buildTree(nums,middle+1,m);
TreeNode * temp=new TreeNode(nums[middle]);
temp->left=left;
temp->right=right;
return temp;

}
TreeNode* sortedArrayToBST(vector<int>& nums) {
int size=nums.size();
TreeNode * res=buildTree(nums,0,size-1);
return res;
}
};