Convert Sorted Array to Binary Search Tree

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

提交一直无法通过，应该是声明TreeNode时候出现的问题。

/**
 * 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 *sortedArrayToBST(vector<int> &num) {
        
        if(num.size() == 0 )return NULL;
        TreeNode * root;
        root = BST(num , 0 ,num.size()-1 );
        return root;
    }
    TreeNode *BST(vector<int> &num, int begin , int end)
    {
        if(begin <= end)
        {
            int mid = (begin + end)/2;
            TreeNode *root = new TreeNode(num[mid]);  
            if(begin == end)return root;
            
            
            root->left = BST(num, begin,mid -1);
            root->right = BST(num, mid +1 ,end);
            return root;
        }
        else return NULL;
        
        
    }
};