# Clone a linked list with next and random pointer in O(1) space

Given a linked list having two pointers in each node. The first one points to the next node of the list, however the other pointer is random and can point to any node of the list. Write a program that clones the given list in O(1) space, i.e., without any extra space.
Examples:

```Input : Head of the below linked list

Output :
A new linked listidentical to the original list.
```

## Recommended: Please try your approach on {IDE} first, before moving on to the solution.

In the previous posts Set-1 and Set-2 various methods are discussed, and O(n) space complexity implementation is also available.

In this post we’ll be implementing an algorithm that’d require no additional space as discussed in Set-1.

Below is the Algorithm:

• Create the copy of node 1 and insert it between node 1 & node 2 in original Linked List, create the copy of 2 and insert it between 2 & 3.. Continue in this fashion, add the copy of N afte the Nth node
• Now copy the random link in this fashion
``` original->next->random= original->random->next;  /*TRAVERSE
TWO NODES*/
```

This works because original->next is nothing but copy of original and Original->random->next is nothing but copy of random.

• Now restore the original and copy linked lists in this fashion in a single loop.
```original->next = original->next->next;
copy->next = copy->next->next;```
• Ensure that original->next is NULL and return the cloned list

Below is the C++ implementation.

```// C++ program to clone a linked list with next
// and arbit pointers in O(n) time
#include <bits/stdc++.h>
using namespace std;

// Structure of linked list Node
struct Node
{
int data;
Node *next,*random;
Node(int x)
{
data = x;
next = random = NULL;
}
};

// Utility function to print the list.
void print(Node *start)
{
Node *ptr = start;
while (ptr)
{
cout << "Data = " << ptr->data << ", Random  = "
<< ptr->random->data << endl;
ptr = ptr->next;
}
}

// This function clones a given linked list
// in O(1) space
Node* clone(Node *start)
{
Node* curr = start, *temp;

// every node of original list
while (curr)
{
temp = curr->next;

// Inserting node
curr->next = new Node(curr->data);
curr->next->next = temp;
curr = temp;
}

curr = start;

// adjust the random pointers of the
while (curr)
{
curr->next->random = curr->random->next;

// move to the next newly added node by
// skipping an original node
curr = curr->next?curr->next->next:curr->next;
}

Node* original = start, *copy = start->next;

// save the start of copied linked list
temp = copy;

// now separate the original list and copied list
while (original && copy)
{
original->next =
original->next? original->next->next : original->next;

copy->next = copy->next?copy->next->next:copy->next;
original = original->next;
copy = copy->next;
}

return temp;
}

// Driver code
int main()
{
Node* start = new Node(1);
start->next = new Node(2);
start->next->next = new Node(3);
start->next->next->next = new Node(4);
start->next->next->next->next = new Node(5);

// 1's random points to 3
start->random = start->next->next;

// 2's random points to 1
start->next->random = start;

// 3's and 4's random points to 5
start->next->next->random =
start->next->next->next->next;
start->next->next->next->random =
start->next->next->next->next;

// 5's random points to 2
start->next->next->next->next->random =
start->next;

cout << "Original list : \n";
print(start);

cout << "\nCloned list : \n";
Node *cloned_list = clone(start);
print(cloned_list);

return 0;
}
```

Output:

```Original list :
Data = 1, Random  = 3
Data = 2, Random  = 1
Data = 3, Random  = 5
Data = 4, Random  = 5
Data = 5, Random  = 2

Cloned list :
Data = 1, Random  = 3
Data = 2, Random  = 1
Data = 3, Random  = 5
Data = 4, Random  = 5
Data = 5, Random  = 2
```

This article is contributed by Ashutosh Kumar 😀 If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.

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