Convert Sorted Array to Binary Search Tree [LeetCode]

Given an array where elements are sorted in ascending order, convert it to a height balanced BST.

Summary: pretty easy, recursive staff.

class Solution {
public:
    TreeNode *sortedArrayToBST(vector<int> &num) {
        int size = num.size();
        if(size == 0) 
            return NULL;
        if(size == 1)
            return new TreeNode(num[0]);
        if(num.size() % 2 == 1) {
            TreeNode * root = new TreeNode(num[size/2]);
            if(size/2 - 1 >= 0){
               vector<int> left(num.begin(), num.begin() + size/2); 
               root->left = sortedArrayToBST(left);
            }
            if(size/2 + 1 < size) {
                vector<int> right(num.begin() + size/2 + 1, num.end());
                root->right = sortedArrayToBST(right);
            }
            return root;
        }else {
            TreeNode * root = new TreeNode(num[size/2 - 1]);
            if(size/2 - 1 - 1 >= 0){
                vector<int> left(num.begin(), num.begin() + size/2 - 1); 
                root->left = sortedArrayToBST(left);
            }
            if(size/2 - 1 + 1 < size) {
                vector<int> right(num.begin() + size/2 -1 + 1, num.end());
                root->right = sortedArrayToBST(right);
            }
            return root;
        }
    }
};