leetcode

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.

个人思路：

1，先遍历一遍链表，创建出复制链表，并且复制链表的next指针已经连接好，同时用一个map存储原结点和新结点的映射关系

2，同时遍历一遍原链表和复制链表，此时关注random指针，根据原结点random指针指向的结点和map表，便可找到对应的复制结点，使复制的random指针指向该复制结点即可

代码：

#include <stddef.h>
#include <map>

using namespace std;

struct RandomListNode
{
    int label;
    RandomListNode *next, *random;
    RandomListNode(int x) : label(x), next(NULL), random(NULL) {}
};

class Solution {
public:
    RandomListNode *copyRandomList(RandomListNode *head) {
        if (!head)
        {
            return NULL;
        }

        map<RandomListNode*, RandomListNode*> original2copy;        
        RandomListNode *originalCurrent = head->next;
        RandomListNode *copyHead = new RandomListNode(head->label);
        RandomListNode *copyCurrent = copyHead;

        original2copy[head] = copyHead;

        while (originalCurrent) //先创建copy结点和连接next指针，并记录配对信息
        {
            copyCurrent->next = new RandomListNode(originalCurrent->label);
            copyCurrent = copyCurrent->next;
            original2copy[originalCurrent] = copyCurrent;
            originalCurrent = originalCurrent->next;
        }

        originalCurrent = head;
        copyCurrent = copyHead;
        while (originalCurrent) //连接random指针
        {
            if (originalCurrent->random)
            {
                copyCurrent->random = original2copy[originalCurrent->random];
            }

            originalCurrent = originalCurrent->next;
            copyCurrent = copyCurrent->next;
        }

        return copyHead;
    }
};

还有一个更加巧妙的思路，将复制结点放在原结点之后，这样一来，复制结点的random指针指向的结点便是原结点random指针指向结点的next结点，最后将原链表和复制链表拆开即可，这个思路比上面那个省去了O(n)的空间消耗

代码：

#include <stddef.h>

struct RandomListNode
{
    int label;
    RandomListNode *next, *random;
    RandomListNode(int x) : label(x), next(NULL), random(NULL) {}
};

class Solution {
public:
    RandomListNode *copyRandomList(RandomListNode *head) {
        if (!head)
        {
            return NULL;
        }

        RandomListNode *current = head;
        RandomListNode *copy = NULL;

        while (current) //创建复制链表，并将复制链表的结点连到原结点之后
        {
            copy = new RandomListNode(current->label);
            copy->next = current->next;
            current->next = copy;
            current = copy->next;
        }

        current = head;
        
        while (current) //设置复制链表结点的random指针
        {
            if (current->random)
            {
                current->next->random = current->random->next;
            }
            current = current->next->next;
        }

        RandomListNode *copyHead = head->next;
        current = head;

        while (current) //将原链表和复制链表拆开
        {
            copy = current->next;
            current->next = copy->next;
            current = current->next;
            if (current)
            {
                copy->next = current->next;
            }
        }

        return copyHead;
    }
};