# 138. Copy List with Random Pointer

#### Medium

***

A linked list of length `n` is given such that each node contains an additional random pointer, which could point to any node in the list, or `null`.

Construct a [**deep copy**](https://en.wikipedia.org/wiki/Object_copying#Deep_copy) of the list. The deep copy should consist of exactly `n` **brand new** nodes, where each new node has its value set to the value of its corresponding original node. Both the `next` and `random` pointer of the new nodes should point to new nodes in the copied list such that the pointers in the original list and copied list represent the same list state. **None of the pointers in the new list should point to nodes in the original list**.

For example, if there are two nodes `X` and `Y` in the original list, where `X.random --> Y`, then for the corresponding two nodes `x` and `y` in the copied list, `x.random --> y`.

Return *the head of the copied linked list*.

The linked list is represented in the input/output as a list of `n` nodes. Each node is represented as a pair of `[val, random_index]` where:

* `val`: an integer representing `Node.val`
* `random_index`: the index of the node (range from `0` to `n-1`) that the `random` pointer points to, or `null` if it does not point to any node.

Your code will **only** be given the `head` of the original linked list.

 

**Example 1:**

![](https://assets.leetcode.com/uploads/2019/12/18/e1.png)

```
Input: head = [[7,null],[13,0],[11,4],[10,2],[1,0]]
Output: [[7,null],[13,0],[11,4],[10,2],[1,0]]
```

**Example 2:**

![](https://assets.leetcode.com/uploads/2019/12/18/e2.png)

```
Input: head = [[1,1],[2,1]]
Output: [[1,1],[2,1]]
```

**Example 3:**

![](https://assets.leetcode.com/uploads/2019/12/18/e3.png)

```
Input: head = [[3,null],[3,0],[3,null]]
Output: [[3,null],[3,0],[3,null]]
```

 

**Constraints:**

* `0 <= n <= 1000`
* `-104 <= Node.val <= 104`
* `Node.random` is `null` or is pointing to some node in the linked list.

```python
"""
# Definition for a Node.
class Node:
    def __init__(self, x: int, next: 'Node' = None, random: 'Node' = None):
        self.val = int(x)
        self.next = next
        self.random = random
"""

class Solution:
    def copyRandomList(self, head: 'Optional[Node]') -> 'Optional[Node]':
        new_head = None
        ptr = head
        prev = None
        while ptr:
            node = Node(ptr.val) # Create Node
            temp = ptr.next # Store next
            ptr.next = node # insert inbetween
            node.next = temp # 
            ptr = temp
        ptr = head
        while ptr and ptr.next:
            ran = ptr.random
            ptr.next.random = ran.next if ran else None
            ptr = ptr.next.next
        ptr = head
        while ptr and ptr.next:
            if new_head is None:
                new_head = ptr.next
            if prev is not None:
                prev.next = ptr.next
            prev = ptr.next
            ptr = ptr.next.next
        return new_head
```