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.

一刷
题解：

iter 1

第一步创建original node和copy的连接，从而可以通过这个找到random.

/**
 * 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) {
    RandomListNode iter = head, next;

    // First round: make copy of each node,
    // and link them together side-by-side in a single list.
    while (iter != null) {
        next = iter.next;

        RandomListNode copy = new RandomListNode(iter.label);
        iter.next = copy;
        copy.next = next;

        iter = next;
    }

    // Second round: assign random pointers for the copy nodes.
    iter = head;
    while (iter != null) {
        if (iter.random != null) {
            iter.next.random = iter.random.next;
        }
        iter = iter.next.next;
    }

    // Third round: restore the original list, and extract the copy list.
    iter = head;
    RandomListNode pseudoHead = new RandomListNode(0);
    RandomListNode copy, copyIter = pseudoHead;

    while (iter != null) {
        next = iter.next.next;

        // extract the copy
        copy = iter.next;
        copyIter.next = copy;
        copyIter = copy;

        // restore the original list
        iter.next = next;

        iter = next;
    }

    return pseudoHead.next;
}
}

二刷
思路同上，注意恢复原list

/**
 * 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) {
    RandomListNode iter = head, next;

    // First round: make copy of each node,
    // and link them together side-by-side in a single list.
    while (iter != null) {
        next = iter.next;

        RandomListNode copy = new RandomListNode(iter.label);
        iter.next = copy;
        copy.next = next;

        iter = next;
    }

    // Second round: assign random pointers for the copy nodes.
    iter = head;
    while (iter != null) {
        if (iter.random != null) {
            iter.next.random = iter.random.next;
        }
        iter = iter.next.next;
    }

    // Third round: restore the original list, and extract the copy list.
    iter = head;
    RandomListNode pseudoHead = new RandomListNode(0);
    RandomListNode copy, copyIter = pseudoHead;

    while (iter != null) {
        next = iter.next.next;

        // extract the copy
        copy = iter.next;
        copyIter.next = copy;
        copyIter = copy;

        // restore the original list
        iter.next = next;

        iter = next;
    }

    return pseudoHead.next;
}
}