convert sorted list to binary search tree

public:
TreeNode *sortedListToBST(ListNode *head) {
    int len = getListLen(head);
    
    if(len == 0) return NULL;
    
    return generateBST(head,0,len-1);
}
private:
int getListLen(ListNode* head){
    int len = 0;
    
    while(head != NULL){
        len++;
        head = head->next;
    }
    
    return len;
}
TreeNode* generateBST(ListNode* head, int l, int r){
    if(l>r) return NULL;
    
    int mid = l + (r-l)/2;
    ListNode* p = head;
    for(int i=l; i<mid; i++){
        p = p->next;
    }
    
    TreeNode* root = new TreeNode(p->val);
    root->left = generateBST(head,l,mid-1);
    root->right = generateBST(p->next,mid+1,r);//相应的需要变化
}
//Approach 2: 自底而上,O(n)
TreeNode* generateBST(ListNode* &head, int l, int r){//引用
    if(l>r) return NULL;
    
    int mid = l + (r-l)/2;
    
    TreeNode* leftNode = generateBST(head,l,mid-1);//返回子链的根
    //head 如何变化?????为何指向链中点
    
    TreeNode* root = new TreeNode(head->val);//没明白
    head = head->next;
    TreeNode* rightNode = generateBST(head,mid+1,r);
    
    root->left = leftNode;
    root->right = rightNode;
}