LeetCode-Convert Sorted Array to Binary Search Tree

Given an array where elements are sorted in ascending order, convert it to a height balanced BST.

/**
 * 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 * CreateNode(vector<int>&vals,int start,int end){
        if(start>end)return NULL;
        int mid=(start+end)/2;
        TreeNode* ret=new TreeNode(vals[mid]);
        ret->left=CreateNode(vals,start,mid-1);
        ret->right=CreateNode(vals,mid+1,end);
    }
    TreeNode *sortedArrayToBST(vector<int> &num) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        return CreateNode(num,0,num.size()-1);
    }
};