leetcode new problems

leetcode/lc23/MergeKLists.cpp

@@ -0,0 +1,34 @@
+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)
+