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