148. Sort List.md

// top down
class Solution {
public:
    ListNode* sortList(ListNode* head) {
        if (!head || !head->next) return head;
        ListNode* slow = head;
        ListNode* fast = head->next;
        while (fast && fast->next) {
            fast = fast->next->next;
            slow = slow->next;
        }
        ListNode* mid = slow->next;
        slow->next = nullptr;
        return merge(sortList(head), sortList(mid));
    }
    
    ListNode* merge(ListNode* l1, ListNode* l2) {
        ListNode dummy(0);
        ListNode* cur = &dummy;
        
        while (l1 && l2) {
            if (l1->val < l2->val) {
                cur->next = l1;
                l1 = l1->next;
            } else {
                cur->next = l2;
                l2 = l2->next;
            }
            cur = cur->next;
        }
        
        if (l1) cur->next = l1;
        if (l2) cur->next = l2;
        
        return dummy.next;
    }
};