_138.java

package com.fishercoder.solutions;

import java.util.HashMap;
import java.util.Map;

/**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.*/

public class _138 {

    public RandomListNode copyRandomList(RandomListNode head) {
        /**Key is the original nodes, value is the new nodes we're deep copying to.*/
        Map<RandomListNode, RandomListNode> map = new HashMap();
        RandomListNode node = head;

        //loop for the first time: copy the node themselves with only labels
        while(node != null){
            map.put(node, new RandomListNode(node.label));
            node = node.next;
        }
        
        //loop for the second time: copy random and next pointers
        node = head;
        while(node != null){
            map.get(node).next = map.get(node.next);
            map.get(node).random = map.get(node.random);
            node = node.next;
        }
        
        return map.get(head);
    }

    // Definition for singly-linked list with a random pointer.
    class RandomListNode {
        int label;
        RandomListNode next, random;

        RandomListNode(int x) {
            this.label = x;
        }
    }
}