[LeetCode] 138. Copy List with Random Pointer 拷贝带随机指针的链表

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.

这道题的难点在于如何处理随机指针，由于每一个节点都有一个随机指针，这个指针可以为空，也可以指向链表的任意一个节点，如果在生成一个新节点给其随机指针赋值时，都去遍历原链表的话，OJ上肯定会超时。建立一个原节点和新节点的HashMap，给随机指针赋值时查找HashMap，这样可缩短查找时间。

解法1: HashMap

解法2: 

Java：

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public RandomListNode copyRandomList(RandomListNode head) {     if (head == null)         return null;     HashMap<RandomListNode, RandomListNode> map = new HashMap<RandomListNode, RandomListNode>();     RandomListNode newHead = new RandomListNode(head.label);        RandomListNode p = head;     RandomListNode q = newHead;     map.put(head, newHead);        p = p.next;     while (p != null) {         RandomListNode temp = new RandomListNode(p.label);         map.put(p, temp);         q.next = temp;         q = temp;         p = p.next;     }        p = head;     q = newHead;     while (p != null) {         if (p.random != null)             q.random = map.get(p.random);         else             q.random = null;            p = p.next;         q = q.next;     }        return newHead; }

Java:

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public RandomListNode copyRandomList(RandomListNode head) {        if (head == null)         return null;        RandomListNode p = head;        // copy every node and insert to list     while (p != null) {         RandomListNode copy = new RandomListNode(p.label);         copy.next = p.next;         p.next = copy;         p = copy.next;     }        // copy random pointer for each new node     p = head;     while (p != null) {         if (p.random != null)             p.next.random = p.random.next;         p = p.next.next;     }        // break list to two     p = head;     RandomListNode newHead = head.next;     while (p != null) {         RandomListNode temp = p.next;         p.next = temp.next;         if (temp.next != null)             temp.next = temp.next.next;         p = p.next;     }        return newHead; }

Python:   Time: O(n)  Space: O(n)

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# Definition for singly-linked list with a random pointer. class RandomListNode:     def __init__(self, x):         self.label = x         self.next = None         self.random = None   class Solution2:     # @param head, a RandomListNode     # @return a RandomListNode     def copyRandomList(self, head):         dummy = RandomListNode(0)         current, prev, copies = head, dummy, {}           while current:             copied = RandomListNode(current.label)             copies[current] = copied             prev.next = copied             prev, current = prev.next, current.next           current = head         while current:             if current.random:                 copies[current].random = copies[current.random]             current = current.next           return dummy.next     if __name__ == "__main__":     head = RandomListNode(1)     head.next = RandomListNode(2)     head.random = head.next     result = Solution().copyRandomList(head)     print(result.label)     print(result.next.label)     print(result.random.label)

Python: Time: O(n)  Space: O(1)

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class Solution:     # @param head, a RandomListNode     # @return a RandomListNode     def copyRandomList(self, head):         # copy and combine copied list with original list         current = head         while current:             copied = RandomListNode(current.label)             copied.next = current.next             current.next = copied             current = copied.next           # update random node in copied list         current = head         while current:             if current.random:                 current.next.random = current.random.next             current = current.next.next           # split copied list from combined one         dummy = RandomListNode(0)         copied_current, current = dummy, head         while current:             copied_current.next = current.next             current.next = current.next.next             copied_current, current = copied_current.next, current.next         return dummy.next

C++:

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class Solution { public:     RandomListNode *copyRandomList(RandomListNode *head) {         if (!head) return NULL;         RandomListNode *res = new RandomListNode(head->label);         RandomListNode *node = res;         RandomListNode *cur = head->next;         map<RandomListNode*, RandomListNode*> m;         m[head] = res;         while (cur) {             RandomListNode *tmp = new RandomListNode(cur->label);             node->next = tmp;             m[cur] = tmp;             node = node->next;             cur = cur->next;         }         node = res;         cur = head;         while (node) {             node->random = m[cur->random];             node = node->next;             cur = cur->next;         }         return res;     } };

 

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