138.Copy List with Random Pointer

Method 1

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] ]

Method 1

【O(n) time | O(1) space】

package Leetcode.LinkList;

/**
 * @author zhengstars
 * @date 2024/10/13
 */
public class CopyListWithRandomPointer {
    public static Node copyRandomList(Node head) {
        if (head == null) {
            return null; 
        }

        // Step 1: Create copy nodes and insert them after original nodes
        Node curr = head;
        while (curr != null) {
            // Create a new node with the same value as the current node
            Node copy = new Node(curr.val);
            // Insert the new node right after the current node
            copy.next = curr.next;
            curr.next = copy;
            // Move to the next original node
            curr = copy.next;
        }

        // Step 2: Set random pointers for the copy nodes
        curr = head;
        while (curr != null) {
            // If the original node has a random pointer, set the copy's random pointer
            if (curr.random != null) {
                // curr.next is the copy of curr
                // curr.random.next is the copy of curr.random
                curr.next.random = curr.random.next;
            }
            // Move to the next original node
            curr = curr.next.next;
        }

        // Step 3: Separate the original and copied lists
        Node dummy = new Node(0); // Dummy node to start the copied list
        Node copyCurr = dummy;
        curr = head;
        while (curr != null) {
            // Connect the copy nodes
            copyCurr.next = curr.next;
            // Restore the original list connection
            curr.next = curr.next.next;
            // Move both pointers
            copyCurr = copyCurr.next;
            curr = curr.next;
        }

        // Return the head of the copied list
        return dummy.next;
    }

    // Test cases
    public static void main(String[] args) {

        // Test case 1: [ [ 7,null],[13,0],[11,4],[10,2],[1,0] ]
        Node head1 = new Node(7);
        Node node13 = new Node(13);
        Node node11 = new Node(11);
        Node node10 = new Node(10);
        Node node1 = new Node(1);
        head1.next = node13;
        node13.next = node11;
        node11.next = node10;
        node10.next = node1;
        head1.random = null;
        node13.random = head1;
        node11.random = node1;
        node10.random = node11;
        node1.random = head1;

        Node result1 = copyRandomList(head1);
        System.out.println("Test case 1 passed: " + validateCopy(head1, result1));

        // Test case 2: [ [ 1,1],[2,1] ]
        Node head2 = new Node(1);
        Node node2 = new Node(2);
        head2.next = node2;
        head2.random = node2;
        node2.random = node2;

        Node result2 = copyRandomList(head2);
        System.out.println("Test case 2 passed: " + validateCopy(head2, result2));

        // Test case 3: [ [ 3,null],[3,0],[3,null] ]
        Node head3 = new Node(3);
        Node node3_2 = new Node(3);
        Node node3_3 = new Node(3);
        head3.next = node3_2;
        node3_2.next = node3_3;
        head3.random = null;
        node3_2.random = head3;
        node3_3.random = null;

        Node result3 = copyRandomList(head3);
        System.out.println("Test case 3 passed: " + validateCopy(head3, result3));
    }

    private static boolean validateCopy(Node original, Node copy) {
        while (original != null && copy != null) {
            if (original.val != copy.val) {
                return false;
            }
            if ((original.random == null && copy.random != null) ||
                    (original.random != null && copy.random == null)) {
                return false;
            }
            if (original.random != null && original.random.val != copy.random.val) {
                return false;
            }
            original = original.next;
            copy = copy.next;
        }
        return original == null && copy == null;
    }
}

class Node {
    int val;
    Node next;
    Node random;

    public Node(int val) {
        this.val = val;
        this.next = null;
        this.random = null;
    }
}

Method 1

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