138. Copy List with Random Pointer

/*
// Definition for a Node.
class Node {
    public int val;
    public Node next;
    public Node random;

    public Node() {}

    public Node(int _val,Node _next,Node _random) {
        val = _val;
        next = _next;
        random = _random;
    }
};
*/
public class Solution {
  // HashMap which holds old nodes as keys and new nodes as its values.
  HashMap<Node, Node> visitedHash = new HashMap<Node, Node>();

  public Node copyRandomList(Node head) {

    if (head == null) {
      return null;
    }

    // If we have already processed the current node, then we simply return the cloned version of
    // it.
    if (this.visitedHash.containsKey(head)) {
      return this.visitedHash.get(head);
    }

    // Create a new node with the value same as old node. (i.e. copy the node)
    Node node = new Node(head.val, null, null);

    // Save this value in the hash map. This is needed since there might be
    // loops during traversal due to randomness of random pointers and this would help us avoid
    // them.
    this.visitedHash.put(head, node);

    // Recursively copy the remaining linked list starting once from the next pointer and then from
    // the random pointer.
    // Thus we have two independent recursive calls.
    // Finally we update the next and random pointers for the new node created.
    node.next = this.copyRandomList(head.next);
    node.random = this.copyRandomList(head.random);

    return node;
  }
}

Runtime: 0 ms, faster than 100.00% of Java online submissions for Copy List with Random Pointer.
Memory Usage: 42.9 MB, less than 6.97% of Java online submissions for Copy List with Random Pointer.

이거 아예 이해안됨...ㅎ
전씨 헲!!!!!