Leetcode 138. Copy List with Random Pointer

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 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:

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:

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

Example 3:

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.

The solving direction is clone the original linked list first, then using hash map to cache the random node mapping, then go through it again and re-link the node.

"""
# 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]':
        clone_head, clone_pointer = None, None

        node_map = {}

        pointer = head

        while pointer is not None:
            if clone_head is None:
                clone_head = Node(pointer.val, None, None)
                clone_pointer = clone_head
            else:
                clone_pointer.next = Node(pointer.val, None, None)
                clone_pointer = clone_pointer.next

            node_map[pointer] = clone_pointer

            pointer = pointer.next

        clone_pointer = clone_head
        
        while head is not None:
            clone_pointer.random = node_map.get(head.random)

            clone_pointer = clone_pointer.next
            head = head.next

        return clone_head