LeetCode 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:
TreeNode *sortedArrayToBST(vector<int> &num) {
int n = num.size();
if (n == 0)
return NULL;

TreeNode *head = new TreeNode(num[(n - 1) / 2]);
if (n - 3 < 0) //注意：当 n == 2 时， （n - 1)/2 和（n - 3）/ 2 都为 0，此时下面不能再创建节点了。
head->left = NULL;
else
head->left = creatBST(num, 0, (n - 3) / 2);
head->right = creatBST(num, (n + 1) / 2, n - 1);

return head;
}

TreeNode *creatBST(vector<int> &num, int left, int right) {
if (right < left)
return NULL;
int mid = (left + right) / 2;
TreeNode *myNode = new TreeNode(num[mid]);
myNode->left = creatBST(num, left, mid - 1);
myNode->right = creatBST(num, mid + 1, right);
return myNode;
}
};