LeetCode:Copy List with Random Pointer

題目地址：here

題目大意：深拷貝一個鏈表，鏈表除了含有next指針外，還包含一個random指針，該指針指向字符串中的某個節點或者為空。

節點定義為：

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

假設原始鏈表如下，細線表示next指針，粗線表示random指針，沒有畫出的指針均指向NULL：

算法1：我們在構建新鏈表的節點時，保存原始鏈表的next指針映射關系，並把指針做如下變化(藍色為原始鏈表節點，紫紅色為新鏈表節點)：

然后在上圖的基礎上進行如下兩步

1、構建新鏈表的random指針：比如new1->random = new1->random->random->next, new2->random = NULL, new3-random = NULL, new4->random = new4->random->random->next

2、恢復原始鏈表：根據最開始保存的原始鏈表next指針映射關系恢復原始鏈表

該算法時間空間復雜度均為O(N)

---

算法2：該算法更為巧妙，不用保存原始鏈表的映射關系，構建新節點時，指針做如下變化，即把新節點插入到相應的舊節點后面：

同理分兩步

1、構建新節點random指針：new1->random = old1->random->next, new2-random = NULL, new3-random = NULL, new4->random = old4->random->next

2、恢復原始鏈表以及構建新鏈表：例如old1->next = old1->next->next,  new1->next = new1->next->next

該算法時間復雜度O(N)，空間復雜度O(1)

---

下面分別為算法1和算法2代碼：

class Solution {
public:
    RandomListNode *copyRandomList(RandomListNode *head)
     {
        // Note: The Solution object is instantiated only once and is reused by each test case.
        if(head == NULL)return NULL;
        std::map<RandomListNode *,RandomListNode *> oldlistMap;
        RandomListNode *result = new RandomListNode(head->label) ;
        RandomListNode *pold = head, *pnew = result;
        pnew->random = pold;
        //pold->next = pnew;
        RandomListNode* poldPre = pold, *pnewPre = pnew;
        while(pold->next != NULL)
        {
            //保存old list的next指針
            oldlistMap.insert(std::map<RandomListNode*,
                              RandomListNode*>::value_type(pold, pold->next));
            pold = pold->next;
            poldPre->next = pnew;
            pnew = new RandomListNode(pold->label);
            pnewPre->next = pnew;
            pnew->random = pold;

            poldPre = pold;
            pnewPre = pnew;
        }
        pold->next = pnew;//設置old list最后一個節點

        //設置new list的random指針
        pnew = result;
        while(pnew != NULL)
        {
            if(pnew->random->random)
                pnew->random = pnew->random->random->next;
            else pnew->random = NULL;
            pnew = pnew->next;
        }
        //恢復old list的next指針
        pold = head;
        for(int i = 1; i <= oldlistMap.size(); i++)
        {
            pold->next = oldlistMap[pold];
            pold = pold->next;
        }
        pold->next = NULL;//old list 最后一個節點

        return result;
    }
};

class Solution {
public:
    RandomListNode *copyRandomList(RandomListNode *head)
     {
        // Note: The Solution object is instantiated only once and is reused by each test case.
        if(head == NULL)return NULL;
        RandomListNode *result = NULL;
        RandomListNode *pold = head, *pnew = result, *poldNext = NULL;
        do
        {
            poldNext = pold->next;
            pnew = new RandomListNode(pold->label);
            pold->next = pnew;
            pnew->next = poldNext;

            if(result == NULL)
                result = pnew;
            pold = poldNext;
        }while(pold);
        //設置new list的random指針
        pold = head;
        while(pold)
        {
            if(pold->random)
                pold->next->random = pold->random->next;
            pold = pold->next->next;
        }
        //恢復old list 和new list
        pold = head;
        pnew = result;
        while(pnew->next)
        {
            pold->next = pnew->next;
            pold = pold->next;
            pnew->next = pold->next;
            pnew = pnew->next;
        }
        pold->next = NULL;
        pnew->next = NULL;
        return result;
    }
};

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