[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.

最开始的想法就是暴力复制，时间复杂度为O(n^2)，写的时候就感觉要出现事，果不其然，超时了，后来网上看到一个O(n)的算法，非常巧妙的利用了原来链表的信息：

该算法更为巧妙，不用保存原始链表的映射关系，构建新节点时，指针做如下变化，即把新节点插入到相应的旧节点后面：

 

 

同理分两步

 

1、构建新节点random指针：new1->random = old1->random->next, new2-random = NULL, new3-random = NULL, new4->random = old4->random->next

 

2、恢复原始链表以及构建新链表：例如old1->next = old1->next->next,  new1->next = new1->next->next

 

该算法时间复杂度O(N)，空间复杂度O(1)

 

/**
 * 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 *copyRandomList(RandomListNode *head) {
        if (head == NULL) return NULL;
        RandomListNode *pos1 = head, *pos2 = head->next;
        while (pos1 != NULL) {
            pos1->next = new RandomListNode(pos1->label);
            pos1->next->next = pos2;
            pos1 = pos2;
            if (pos2 != NULL)
                pos2 = pos2->next;
        }
        pos1 = head;  pos2 = head->next;
        while (pos1 != NULL) {
            if (pos1->random == NULL) {
                pos2->random = NULL;
            } else {
                pos2->random = pos1->random->next;
            }
            pos1 = pos1->next->next;
            if (pos2->next != NULL)
                pos2 = pos2->next->next;
        }
        RandomListNode *res = head->next;
        pos1 = head; pos2 = head->next;
        while(pos2->next != NULL) {
            pos1->next = pos2->next;
            pos1 = pos2;
            if (pos2->next != NULL)
                pos2 = pos2->next;
        }
        pos1->next = NULL;
        pos2->next = NULL;
        return res;
    }
};

下面是最开始的超时的暴力复制代码：

/**
 * 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 *copyRandomList(RandomListNode *head) {
        if (head == NULL) return NULL;
        RandomListNode *res = new RandomListNode(head->label);
        RandomListNode *pos1 = head->next, *pos2 = res;
        while (pos1 != NULL) {
            pos2->next = new RandomListNode(pos1->label);
            pos1 = pos1->next;
            pos2 = pos2->next;
        }
        pos1 = head;  pos2 = res;
        RandomListNode *idx1 = head, *idx2 = res;
        while (pos1 != NULL) {
            if (pos1->random == NULL) {
                pos2->random = NULL;
            } else {
                idx1 = head; idx2 = res;
                while (pos1->random != idx1) {
                    idx1 = idx1->next;
                    idx2 = idx2->next;
                }
                pos2->random = idx2;
            }
            pos1 = pos1->next;
            pos2 = pos2->next;
        }
        return res;
    }
};