My solution using merge sort


  • 0
    Y
    ListNode *mergeTwoLists(ListNode *l1, ListNode *l2) {
    	if (l1 == nullptr) return l2;
    	if (l2 == nullptr) return l1;
    
    	if (l1->val < l2->val) {
    		l1->next = mergeTwoLists(l1->next, l2);
    		return l1;
    	}
    	else {
    		l2->next = mergeTwoLists(l2->next, l1);
    		return l2;
    	}
    }
    
    ListNode* mergeSort(vector<ListNode*>& lists, int low, int high) {
    	if (low == high) {
    		return lists[low];
    	}
    	if (low < high) {
    		int middle = (low + high) / 2;
    		ListNode *first = mergeSort(lists, low, middle);
    		ListNode *second = mergeSort(lists, middle+1, high);
    		return mergeTwoLists(first, second);
    	}
    }
    
    ListNode* mergeKLists(vector<ListNode*>& lists) {
    	if (lists.size() == 0) {
    		return nullptr;
    	}
    	return mergeSort(lists, 0, lists.size() - 1);
    }
    

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