Convert Sorted Array to Binary Search Tree

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
/**
 * 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 *build(vector<int> &num, int l, int r) {
        if (l <= r)
        {
            int m = (l+r)/2;
            TreeNode *root = new TreeNode(num[m]);
            root->left = build(num,l,m-1);
            root->right = build(num,m+1,r);
            return root;
        }
        else
        {
            return NULL;
        }
        
    }
    TreeNode *sortedArrayToBST(vector<int> &num) {
        int n = num.size();
        if (n == 0)
            return 0;
        return build(num,0,n-1);
    }
};