Merge pull request #557 from sr-sweta/Contribute

Reverse Nodes in k-Groups using CPP

Author: smv1999

Data Structures/Linked Lists/Singly Linked List/Reverse_Nodes_in_K_Groups.cpp

@@ -0,0 +1,94 @@
+/*Given a linked list, reverse the nodes of a linked list k at a time and return its modified list.
+k is a positive integer and is less than or equal to the length of the linked list. If the number of nodes is not a multiple of k then left-out nodes, in the end, should remain as it is.
+You may not alter the values in the list's nodes, only nodes themselves may be changed.
+
+Example :
+
+		1 ---> 2 ---> 3 ---> 4 ---> 5
+	
+	becomes
+	 
+	 	2 ---> 1 ---> 4 ---> 3 ---> 5
+					
+		
+Input: head = [1,2,3,4,5], k = 2
+Output: [2,1,4,3,5]
+
+Follow-up: Can you solve the problem in O(1) extra memory space?
+*/
+
+#include <bits/stdc++.h>
+using namespace std;
+
+struct ListNode
+{
+    int val;
+    ListNode *next;
+    ListNode(int n)
+    {
+        val = n;
+        next = NULL;
+    }
+};
+
+void printList(ListNode* head){
+	int i = 0;
+	while(head){
+		cout<<head->val<<" ";
+		head = head->next;
+	}
+}
+
+ListNode* reverseKGroup(ListNode* head, int k) {
+    ListNode* curr = head;
+    int count = 0;
+    
+    //Take first k group of nodes
+    while (curr != NULL && count != k) { 
+        curr = curr->next;
+        count++;
+    }
+    
+    if (count == k) { 
+	    //Recursively taking all k group of nodes
+	    //Passing next group first node as curr
+        curr = reverseKGroup(curr, k); 
+        
+        //Go on reversing the node group using normal reverse steps
+        while (count-- > 0) { 
+            ListNode* tmp = head->next; 
+            head->next = curr;
+            curr = head; 
+            head = tmp; 
+        }
+        head = curr;
+    }
+    return head;
+}
+    
+int main()
+{
+    int NumOfNodes, nn, data, k;
+    cin>>NumOfNodes;
+    nn = NumOfNodes;
+    ListNode* head = NULL;
+    ListNode* tail = head;
+    
+    while(nn--){
+    	cin>>data;
+    	if(!head){
+    		head = new ListNode(data);
+    		tail = head;
+		}
+		else{
+			tail->next = new ListNode(data);
+			tail = tail->next;
+		}			
+	}
+	cin>>k;
+	ListNode* finalHead = reverseKGroup(head, k);
+	printList(finalHead);
+    return 0;
+}
+//No extra space used.
+