109. Convert Sorted List to Binary Search Tree

class Solution {
public:
    TreeNode* sortedListToBST(ListNode* head) {
        return buildBST(head);
    }
    
    TreeNode *buildBST(ListNode *head){ // mid as root
        if (head==NULL) return NULL;
        if (head->next==NULL) return new TreeNode(head->val);
        
        ListNode *slow=head, *fast=head, *prev;
        while (fast!=NULL && fast->next!=NULL){
            fast = fast->next->next;
            prev = slow;
            slow = slow->next;
        }
        prev->next = NULL;
        ListNode *mid=slow;
        TreeNode *root=new TreeNode(mid->val);
        root->left  = buildBST(head); 
        root->right = buildBST(mid->next);
        return root;
    }
};