[Leetcode] Copy list with random pointer

A linked list is given such that each node contains an additional random pointer which cocould point to any node in the list or null.

Return a deep copy of the list.

Idea: the first time the linked list is copied normally, and the hash table is used to save the correspondence between the original nodes and the new nodes in the linked list. The second time the traversal chain table is used, the random field is copied.

This is a typical practice of changing the space time. For N nodes, the hash table with the size of O (n) is required, and the time complexity can be reduced to O (n ).

/** * 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) {        if (head == null) {return head;}HashMap<RandomListNode, RandomListNode> map = new HashMap<RandomListNode, RandomListNode>();RandomListNode res = null;RandomListNode taiListNode = null;RandomListNode cur = head;while (cur != null) {if (res == null) {res = new RandomListNode(cur.label);res.next = res.random = null;taiListNode = res;map.put(head, res);}else{taiListNode.next = new RandomListNode(cur.label);taiListNode = taiListNode.next;map.put(cur, taiListNode);}cur = cur.next;}taiListNode.next = null;cur = head;taiListNode = res;while (cur != null) {taiListNode.random = (RandomListNode)map.get((RandomListNode)cur.random);cur = cur.next;taiListNode = taiListNode.next;}return res;    }}