[LeetCode] 138. Copy List with Random Pointer

A linked list is given such that each node contains an additional random pointer which could point to any node in the list or null.
Return a deep copy of the list.

Thought process:
The problem is that, as we iterate through the list, current.random may not have been created. One solution is to iterate through the list twice and use a hashmap. 
  1. First iteration: copy nodes' values, next pointers, and map old node -> new node.
  2. Second iteration: copy random pointers.
Solution 1:
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/**
 * Definition for singly-linked list with a random pointer.
 * class RandomListNode {
 *     int label;
 *     RandomListNode next, random;
 *     RandomListNode(int x) { this.label = x; }
 * };
 */
public class Solution {
    public RandomListNode copyRandomList(RandomListNode head) {
        if (head == null) {
            return null;
        }
        
        RandomListNode headCopy = new RandomListNode(head.label);
        RandomListNode curr1 = head;
        RandomListNode curr2 = headCopy;
        
        Map<RandomListNode, RandomListNode> map = new HashMap<>();
        map.put(curr1, curr2);
        
        while (curr1.next != null) {
            RandomListNode copy = new RandomListNode(curr1.next.label);
            curr2.next = copy;
            curr1 = curr1.next;
            curr2 = curr2.next;
            map.put(curr1, curr2);
        }
        
        curr1 = head;
        while (curr1 != null) {
            map.get(curr1).random = map.get(curr1.random);
            curr1 = curr1.next;
        }
        
        return headCopy;
    }
}

Time complexity: O(n).

Can we solve this without using a hashmap?
Yes. The trick is to put a node's copy at node.next, and split the list afterwards.

Solution 2:

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/**
 * Definition for singly-linked list with a random pointer.
 * class RandomListNode {
 *     int label;
 *     RandomListNode next, random;
 *     RandomListNode(int x) { this.label = x; }
 * };
 */
public class Solution {
    public RandomListNode copyRandomList(RandomListNode head) {
        if (head == null) {
            return null;
        }
        
        RandomListNode current = head;
        while (current != null) {
            RandomListNode next = current.next;
            current.next = new RandomListNode(current.label);
            current.next.next = next;
            current = next;
        }
        
        current = head;
        while (current != null) {
            if (current.random != null) {
                current.next.random = current.random.next;
            }
            current = current.next.next;
        }
        
        current = head;
        RandomListNode headCopy = current.next;
        RandomListNode currCopy = headCopy;
        while (current != null) {
            current.next = current.next.next;
            if (currCopy.next != null) {
                currCopy.next = currCopy.next.next;
            }
            current = current.next;
            currCopy = currCopy.next;
        }
        
        return headCopy;
    }
}
Time complexity: O(n).
Location: San Jose, CA, USA

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