138. Copy List with Random Pointer

题目：

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.

链接：https://leetcode.com/problems/copy-list-with-random-pointer/#/description

4/16/2017

自己想不出来，答案是抄的。

对于链表的题，需要把递归当成一种常见的思路！而且，一定要考虑到输入的各种陷阱！比如此题，map里一定要存旧的和新的node而不是旧label新node。最重要的是，链表一定要想到是否有circle！一开始的解法对circle不适用我觉得不是大问题，然而，一定要沟通出来，比方说在leetcode答案看下来之后，只有这种方法是对circle有效的。

/**
 * 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 {
    Map<RandomListNode, RandomListNode> visited = new HashMap<>();

    public RandomListNode copyRandomList(RandomListNode head) {
        if(head == null)
            return head;
        if(visited.containsKey(head))
            return visited.get(head);
        RandomListNode root = new RandomListNode(head.label);
        visited.put(head, root);
        
        root.next = copyRandomList(head.next);
        root.random = copyRandomList(head.random);
        return root;
    }
}

其他的各种好思路，然而，只对于没有circle的list有用：

这个解法是将旧的和新的放在一起，通过三次遍历重塑两个list，充分利用了next的特性

https://discuss.leetcode.com/topic/7594/a-solution-with-constant-space-complexity-o-1-and-linear-time-complexity-o-n

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;
}

另外一个，先全部建立新node，再到map中找

https://discuss.leetcode.com/topic/18086/java-o-n-solution

public RandomListNode copyRandomList(RandomListNode head) {
  if (head == null) return null;
  
  Map<RandomListNode, RandomListNode> map = new HashMap<RandomListNode, RandomListNode>();
  
  // loop 1. copy all the nodes
  RandomListNode node = head;
  while (node != null) {
    map.put(node, new RandomListNode(node.label));
    node = node.next;
  }
  
  // loop 2. assign next and random 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);
}

更多讨论：

https://discuss.leetcode.com/category/146/copy-list-with-random-pointer