Create 25. Reverse Nodes in k-Group.cpp

It is a C++ code snippet solution for leetcode question 25. Reverse Nodes in k-Group (Hard). It is solved using three pointers: previous, current & next. It uses an iterative approach.

Author: gauravabhishek4U

25. Reverse Nodes in k-Group.cpp

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+class Solution {
+public:
+    void reverse(ListNode *start, ListNode *end)
+    {
+        ListNode *prev = NULL;
+        ListNode *curr = start;
+        ListNode *nexty = start->next;
+        
+        while(prev!=end)
+        {
+            curr->next = prev;
+            prev = curr;
+            curr = nexty;
+            
+            if(nexty!=NULL)
+            {
+                nexty = nexty->next;
+            }
+        }
+        
+    }
+    
+    ListNode* reverseKGroup(ListNode* head, int k) {
+        
+        if(head==NULL || head->next == NULL || k==1)
+            return head;
+        
+        ListNode *dummy = new ListNode(-1);
+        dummy->next = head;
+        ListNode *beforeStart = dummy, *end = head;
+        int i=0;
+        while(end!=NULL)
+        {
+            i++;
+            if(i%k==0)
+            {
+                //reversal
+                ListNode *start = beforeStart->next, *temp = end->next;
+                reverse(start,end);
+                beforeStart->next = end;
+                start->next = temp;
+                beforeStart = start;
+                end = temp;
+            }
+            else
+            {
+                end = end->next;
+            }
+            
+        }
+        return dummy->next;
+    }
+};