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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 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]]

Example 4:

Input: head = []

Output: []

Explanation: The given linked list is empty (null pointer), so return null.

Constraints:

• 0 <= n <= 1000
• -10000 <= Node.val <= 10000
• Node.random is null or is pointing to some node in the linked list.

Solution

require_relative '../../com_github_leetcode/random/node'

# Definition for Node.
# class Node
#     attr_accessor :val, :next, :random
#     def initialize(val = 0)
#         @val = val
#		  @next = nil
#		  @random = nil
#     end
# end

# @param {Node} node
# @return {Node}
def copyRandomList(head)
  return nil if head.nil?

  # First pass to create cloned nodes and insert them after the original nodes
  curr = head
  while curr
    cloned_node = Node.new(curr.val)
    cloned_node.next = curr.next
    curr.next = cloned_node
    curr = cloned_node.next
  end

  # Second pass to set the random pointers of the cloned nodes
  curr = head
  while curr
    curr.next.random = curr.random.next if curr.random
    curr = curr.next.next
  end

  # Third pass to separate the original and cloned nodes
  curr = head
  new_head = nil
  while curr
    cloned_node = curr.next
    new_head ||= cloned_node
    curr.next = cloned_node.next
    cloned_node.next = curr.next.next if curr.next
    curr = curr.next
  end

  new_head
end

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