LeetCode Copy List with Random Pointer

原鏈接

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.

思路分析：這題要求拷貝鏈表，包括內容，next指針和random指針。容易想到的思路是定義一個hashmap保存舊鏈表節點到新鏈表節點的映射關係，第一次遍歷鏈表，只拷貝next指針和node，並且init hashmap， 第二遍遍歷鏈表再拷貝所有節點的random指針,藉助hashmap可以在O（1）時間內由給定舊鏈表節點找到新鏈表對應節點。時間複雜度和空間複雜度都是O（n）。

另有一個解法不需要額外空間，需要三遍遍歷，要仔細畫圖分析，詳細可見

http://codeganker.blogspot.com/2014/03/copy-list-with-random-pointer-leetcode.html

http://blog.csdn.net/fightforyourdream/article/details/16879561

AC Code

/**
 * 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) {
        //0133
        if(head == null) return null;
        RandomListNode newHead = new RandomListNode(head.label);
        HashMap<RandomListNode, RandomListNode> oldToNewMap = new HashMap<RandomListNode, RandomListNode>();
        oldToNewMap.put(head, newHead);
        RandomListNode newCur = newHead; // iter for newList
        RandomListNode cur = head.next; // iter for oriList

        while(cur != null){
            RandomListNode newNode = new RandomListNode(cur.label);
            newCur.next = newNode;
            oldToNewMap.put(cur, newNode);
            newCur = newNode;
            cur = cur.next;
        }

        newCur = newHead;
        cur = head;
        while(cur != null){
            newCur.random = oldToNewMap.get(cur.random);
            cur = cur.next;
            newCur = newCur.next;
        }
        return newHead;
    }
    //0202
}