[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) {}
 * };
 */
class Solution {
    public:
        TreeNode *sortedArrayToBST(vector<int> &num)
        {   
            int size = num.size();
            if(size == 0)
                return NULL;
             return sortedArrayToBSTInternal(num, 0, size - 1); 

        }   

        TreeNode *sortedArrayToBSTInternal(vector<int> &num, int low, int high)
        {   

            // the code is very important, i.e: low = 4, hight = 5, mid = 4, 
            // will call sortedArrayToBSTInternal(num, 4, 3)
            if(low > high)
                return NULL;
            if(low == high)
                return new TreeNode(num[low]); 

            int mid = (high-low)/2 + low;
           
            TreeNode *root = new TreeNode(num[mid]); 
            TreeNode *left = sortedArrayToBSTInternal(num, low, mid - 1); 
            TreeNode *right = sortedArrayToBSTInternal(num, mid + 1, high);
            root->left = left;
            root->right= right;
            return root;
        }   
};