TreeNode* cbst(vector<int>& num, int start, int end) {

if (start > end) return NULL;

int mid = start + (end - start) /2;

TreeNode* root = new TreeNode(num[mid]);

root->left = cbst(num, start, mid-1);

root->right = cbst(num, mid +1, end);

return root;

}

TreeNode *sortedArrayToBST(vector<int> &num) {

if (num.empty()) return NULL;

return cbst(num, 0, num.size()-1);

}