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) {
        return addTreeNode(num, 0, num.size() - 1);
    }
    TreeNode* addTreeNode(vector<int>& num, int low, int high) {
        if (low > high) return nullptr;
        int mid = low + (high - low) / 2;
        TreeNode* root = new TreeNode(num.at(mid));
        root->left  = addTreeNode(num, low, mid - 1);
        root->right = addTreeNode(num, mid + 1, high);
        return root;
    }
};