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 trans(num, 0, num.size() - 1);
    }
    
    TreeNode *trans(vector<int> &num, int left, int right)
    {
        if (left > right)
        {
            return NULL;
        }
        int mid = (right + left) >> 1;
        TreeNode *ptr = new TreeNode(num[mid]);
        ptr->left = trans(num, left, mid - 1);
        ptr->right = trans(num, mid + 1, right);
        return ptr;
    }
};