Optimal Single Traversal Code.

ListNode* reverse(ListNode* head, ListNode* prev) {
	if (head == NULL)
        return NULL;
 
    ListNode* temp;
    ListNode* curr = head;
 
    // Reversing nodes until curr node's value
    // turn odd or Linked list is fully traversed
    while (curr != NULL && curr->val % 2 == 0) {
        temp = curr->next;
        curr->next = prev;
        prev = curr;
        curr = temp;
    }
 
    // If elements were reversed then head
    // pointer needs to be changed
    if (curr != head) {
 
        head->next = curr;
  
        // Recur for the remaining linked list
        curr = reverse(curr, NULL);
        return prev;
    }
 
    // Simply iterate over the odd value nodes
    
        head->next = reverse(head->next, head);
        return head;
    
}
ListNode * reverseEvenElements(ListNode *head) {
	return reverse(head, NULL);
}