Convert Sorted Array to Binary Search Tree

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    TreeNode* sortedArrayToBST(vector<int>& nums,int start,int end){
        if(end <= start) return NULL;
        int mid = start + (end-start)/2;
        TreeNode* root = new TreeNode(nums[mid]);
        root->left = sortedArrayToBST(nums,start,mid);
        root->right = sortedArrayToBST(nums,mid+1,end);
        return root;
    }
    TreeNode* sortedArrayToBST(vector<int>& nums) {
        int nums_size = nums.size();
        return sortedArrayToBST(nums,0,nums_size);
    }
};