code for reverse nodes in k groups

Example 1:
Input: head = [1,2,3,4,5], k = 2
Output: [2,1,4,3,5]

Example 2:
Input: head = [1,2,3,4,5], k = 3
Output: [3,2,1,4,5]

algorithms/CPlusPlus/Linked-Lists/Reverse_nodes_in_k_groups.cpp

@@ -0,0 +1,33 @@
+/**
+ * Definition for singly-linked list.
+ * struct ListNode {
+ *     int val;
+ *     ListNode *next;
+ *     ListNode() : val(0), next(nullptr) {}
+ *     ListNode(int x) : val(x), next(nullptr) {}
+ *     ListNode(int x, ListNode *next) : val(x), next(next) {}
+ * };
+ */
+class Solution {
+public:
+    int length(ListNode *head){
+        int len=0;
+        while(head){
+            len++;
+            head=head->next;
+        }
+        return len;
+    }
+    ListNode* reverseKGroup(ListNode* head, int k) {
+        if(length(head)<k)   return head;
+        ListNode *prev=NULL,*curr=head;
+        for(int i=0;i<k;i++){
+            ListNode *n=curr->next;
+            curr->next=prev;
+            prev=curr;
+            curr=n;   
+        }
+        head->next=reverseKGroup(curr,k);
+        return prev;
+    }
+};