Convert Sorted Array to Binary Search Tree

Given an array where elements are sorted in ascending order, convert it to a height balanced BST.

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