Definition for singly-linked list struct ListNode int val; ListNode next; ListNode val0 nextnullptr ListNodeint x valx nextnullptr ListNodeint x ListNode next valx next

The error in line 15 is because you are trying to access a member variable val and next of a ListNode object a without initializing it. To fix the error, you need to create a new ListNode object and assign the value from the sorted forward_list to it. Here's the corrected code:

class Solution {
public:
    ListNode* sortList(ListNode* head) {
        forward_list<int> l;
        for(auto i=head;i!=nullptr;i=i->next) l.emplace_front(i->val);
        l.sort();
        ListNode* dummy = new ListNode();
        ListNode* a = dummy;
        for(auto i=l.begin();i!=l.end();i++) {
            a->next = new ListNode(*i);
            a = a->next;
        }
        return dummy->next;
    }
};

Note that I've added a dummy ListNode at the beginning to simplify the implementation. The sorted list is stored starting from the next node of the dummy node, and the dummy node itself is not included in the final result