[LeetCode]Copy List with Random Pointer

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.

思考：先进行单链表复制，在定义random指针，每个结点的random指针都要遍历一次链表，时间复杂度为O(n²)。

/**

 * Definition for singly-linked list with a random pointer.

 * struct RandomListNode {

 *     int label;

 *     RandomListNode *next, *random;

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

 * };

 */

class Solution {

public:

	RandomListNode *copyList(RandomListNode *head)

	{

		if(head==NULL) return head;

		RandomListNode *newhead=NULL;

		RandomListNode *p=head;

		RandomListNode *q=newhead;

		while(p)

		{

			RandomListNode *node=new RandomListNode(NULL);

			node->label=p->label;

			if(newhead==NULL)

			{

				newhead=node;

				q=node;

			}

			else

			{

				q->next=node;

				q=q->next;

			}

			p=p->next;

		}

		return newhead;

	}

	void copyRandomPointer(RandomListNode *head,RandomListNode *newhead)

	{

		if(head==NULL||newhead==NULL) return;

		RandomListNode *p=head;

		RandomListNode *q=newhead;

		RandomListNode *a,*b;

		while(p)

		{

			if(p->random==NULL) 

				q->random==NULL;

			else

			{

				a=head;b=newhead;

				while(p->random!=a)

				{

					a=a->next;

					b=b->next;

				}

				q->random=b;

			}

			p=p->next;

			q=q->next;

		}

	}

    RandomListNode *copyRandomList(RandomListNode *head) {

        // IMPORTANT: Please reset any member data you declared, as

        // the same Solution instance will be reused for each test case.

        if(head==NULL) return head;

		RandomListNode *newhead=copyList(head);

		copyRandomPointer(head,newhead);

		return newhead;

    }

};

　　上述方法，时间都耗费在寻找random指针上。怎样在O(1)时间里找到random指向结点，是本题需要精髓所在。参考这里。

/**

 * Definition for singly-linked list with a random pointer.

 * struct RandomListNode {

 *     int label;

 *     RandomListNode *next, *random;

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

 * };

 */

class Solution {

public:

	void CloneNodes(RandomListNode *head)

	{

		RandomListNode *p=head;

		while(p)

		{

			RandomListNode *node=new RandomListNode(p->label);

			node->next=p->next;

			p->next=node;

			p=node->next;

		}

	}

	void CopyRandomPointer(RandomListNode *head)

	{

		RandomListNode *p=head;

		while(p)

		{

			if(p->random==NULL) 

				p->next->random=NULL;

			else

				p->next->random=p->random->next;

			p=p->next->next;

		}

	}

	RandomListNode *SeparateList(RandomListNode *head)

	{

		RandomListNode *p=head;

		RandomListNode *newhead=NULL;		

		RandomListNode *q=newhead;

		while(p)

		{

			if(newhead==NULL)

			{

				newhead=q=p->next;

				p->next=q->next;

                p=p->next;

			}

			else

			{

				q->next=p->next;

				q=q->next;

				p->next=q->next;

				p=p->next;

			}

		}

		return newhead;

	}

    RandomListNode *copyRandomList(RandomListNode *head) {

        // IMPORTANT: Please reset any member data you declared, as

        // the same Solution instance will be reused for each test case.

        if(head==NULL) return head;

		CloneNodes(head);

		CopyRandomPointer(head);		

		return SeparateList(head);

    }

};

　　总感觉百度后好像失去了什么。。学习解题思路，而不是解题方法，切记！