109 Convert Sorted List to Binary Search Tree

struct TreeNode* sortedListToBST(struct ListNode* head) {

    
    if(head == NULL)
        return NULL;
    if(head->next == NULL){
        struct TreeNode *root = calloc(1, sizeof(struct TreeNode));
        root->val = head->val;
        return root;
    }
    struct ListNode *fast, *slow, *prev;
    struct ListNode *dummy = calloc(1, sizeof(struct ListNode));
    dummy->next = head;
    prev = dummy;
    slow = fast = head;
    while(fast->next){
        fast = fast->next;
        if(fast->next)
            fast = fast->next;
        slow = slow->next;
        prev = prev->next;
    }
    //
 
    //slow is mid pointer
    struct TreeNode *root = calloc(1, sizeof(struct TreeNode));
    root->val = slow->val;
    root->right = sortedListToBST(slow->next);
    prev->next = NULL;
    root->left = sortedListToBST(dummy->next);
 

    free(dummy);

    return root;    
}