dsa-competitive-programming/leetcode/lc23/MergeKLists.cpp

class Solution{
    public : 
        ListNode* mergeTwoLists(ListNode* l1, ListNode* l2){
            if(!l1 || !l2) return l1?l1:l2;
            if(l1->val < l2->val){
                l1->next = mergeTwoLists(l1->next, l2);
                return l1;
            }else{
                l2->next = mergeTwoLists(l1,l2->next);
                return l2;
            }
        }

        ListNode* mergeKLists(vector<ListNode*> &lists){
            if(lists.empty()) return nullptr;
            return divideAndConquer(lists, 0 , lists.size()-1);
        }
        ListNode* divideAndConquer(vector<ListNode*> &lists, int left, int right){
            if(left == right) return lists[left];
            
            int mid = left + (right-left)/2;    
            ListNode* l1 = divideAndConquer(lists, left, mid);
            ListNode* l2 = divideAndConquer(lists, mid+1, right);
            return mergeTwoLists(l1,l2);
        }
};

//We recursively split the K lists into smaller groups, merge pairs of lists at the bottom, and keep merging upward until one sorted list remains.

//TC: O(nlogk) , Each level merges all nodes once, Number of levels = log₂(k), naive merging takes O(nk)


//SC : O(logK)