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) {}
 * };
 */
struct TreeNode
{
	int val;
	TreeNode *left;
	TreeNode *right;
	TreeNode(int x) : val(x), left(NULL), right(NULL) {}
};
class Solution {
public:
    TreeNode *sortedArrayToBST(std::vector<int> &num) {
        return ArrayToBST(num,0,num.size()-1);
    }
private:
	TreeNode *ArrayToBST(std::vector<int> &num, int start, int end)
	{
		if(start > end) return NULL;
		int mid = (start + end) / 2;
		TreeNode *root = new TreeNode(num[mid]);
		root->left = ArrayToBST(num,start,mid - 1);
		root->right = ArrayToBST(num,mid + 1, end);
		return root;
	}
};