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