108.Convert Sorted Array to Binary Search Tree

class Solution {
public:
    TreeNode* sortedArrayToBST(vector<int>& nums) {
        return helper(nums, 0 , (int)nums.size() - 1);
    }
    TreeNode* helper(vector<int>& nums, int left, int right) {
        if (left > right) return NULL;
        int mid = left + (right - left) / 2;
        TreeNode *cur = new TreeNode(nums[mid]);
        cur->left = helper(nums, left, mid - 1);
        cur->right = helper(nums, mid + 1, right);
        return cur;
    }
};