Rotate List in C++

Suppose we have a linked list. We have to rotate the list to the right k places. The value of k is not negative. So if the list is like [1,23,4,5,NULL], and k = 2, then the output will be [4,5,1,2,3,NULL]

Let us see the steps −

If the list is empty, then return null
len := 1
create one node called tail := head
while next of tail is not null
- increase len by 1
- tail := next of tail
next of tail := head
k := k mod len
newHead := null
for i := 0 to len – k
- tail := next of tail
newHead := next of tail
next of tail := null
return newHead

Let us see the following implementation to get better understanding −

Example

Live Demo

#include <bits/stdc++.h>
using namespace std;
class ListNode{
   public:
      int val;
      ListNode *next;
      ListNode(int data){
      val = data;
      next = NULL;
   }
};
ListNode *make_list(vector<int> v){
   ListNode *head = new ListNode(v[0]);
   for(int i = 1; i<v.size(); i++){
      ListNode *ptr = head;
      while(ptr->next != NULL){
         ptr = ptr->next;
      }
      ptr->next = new ListNode(v[i]);
   }
   return head;
}
void print_list(ListNode *head){
   ListNode *ptr = head;
   cout << "[";
   while(ptr->next){
      cout << ptr->val << ", ";
      ptr = ptr->next;
   }
   cout << "]" << endl;
}
class Solution {
   public:
   ListNode* rotateRight(ListNode* head, int k) {
      if(!head) return head;
      int len = 1;
      ListNode* tail = head;
      while(tail->next){
         len++;
         tail = tail->next;
      }
      tail->next = head;
      k %= len;
      ListNode* newHead = NULL;
      for(int i = 0; i < len - k; i++){
         tail = tail->next;
      }
      newHead = tail->next;
      tail->next = NULL;
      return newHead;
   }
};
main(){
   Solution ob;
   vector<int> v = {1,2,3,4,5,6,7,8,9};
   ListNode *head = make_list(v);
   print_list(ob.rotateRight(head, 4));
}