Merge pull request #123 from SaimantikaBasu/patch-1

Update 23.cpp

Author: larissalages

leetcode/cpp/linkedlist/23.cpp

@@ -10,32 +10,48 @@
  */
 class Solution {
 public:
-    ListNode* merge_two(ListNode* l1,ListNode* l2){
-        ListNode* res=NULL;
-        if(!l1) return l2;
-        if(!l2) return l1;
-        if(l1->val<=l2->val){
-            res=l1;
-            res->next=merge_two(l1->next,l2);
-        }else{
-            res=l2;
-            res->next=merge_two(l1,l2->next);
+   ListNode * mergeTwoLL(ListNode * head1 , ListNode * head2){
+    if(head1 == nullptr || head2 == nullptr)
+       return head1==nullptr?head2:head1;
+    ListNode* curr1 = head1;
+    ListNode* curr2 = head2;
+    ListNode* head = new ListNode(-1);
+    ListNode* curr = head;
+
+    while(curr1!= nullptr && curr2!= nullptr){
+        if(curr1->val <= curr2->val){
+             curr->next = curr1;
+             curr1 = curr1->next;
+             curr = curr->next;
+        }
+        else{
+            curr->next = curr2;
+             curr2 = curr2->next;
+             curr = curr->next;
         }
-        return res;
     }
+    if(curr1==nullptr)
+       curr->next = curr2;
+    else if(curr2 == nullptr)
+       curr->next = curr1;
 
+    return head->next;
+}
+    ListNode* mergeKLL(vector<ListNode*> lists,int si ,int ei)
+    {
+        if(si==ei)
+            return lists[si];
+        if(si+1==ei)
+            return mergeTwoLL(lists[si],lists[ei]);
+        
+        int mid=(si+ei)/2;
+        return mergeTwoLL(mergeKLL(lists,si,mid),mergeKLL(lists,mid+1,ei));
+    }
     ListNode* mergeKLists(vector<ListNode*>& lists) {
-       if(lists.empty()) return NULL;
-       int last=lists.size()-1,i,j;
-        while(last!=0){
-            i=0;j=last;
-            while(i<j){
-                lists[i]=merge_two(lists[i],lists[j]);
-                i++;
-                j--;
-            }
-            last=j;
-        }
-        return lists[0];
+        if(lists.size()==0)
+            return NULL;
+        int si=0;int ei=lists.size()-1;
+        
+        return mergeKLL(lists,si,ei);
     }
-};
+};