题目:
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.
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 ) return head; std::map<RandomListNode *, RandomListNode *> oriNext; RandomListNode *pOri = head; RandomListNode dummy(-1); RandomListNode *pCopy = &dummy; // first round visit while (pOri) { // store the original Node's next pointer oriNext[pOri] = pOri->next; // create copy Node pOri->next = new RandomListNode(pOri->label); pCopy->next = pOri->next; pCopy->next->random = pOri; pCopy = pCopy->next; // move to the next Node pOri = oriNext[pOri]; } // second round visit pCopy = dummy.next; while (pCopy) { pCopy->random = pCopy->random->random ? pCopy->random->random->next : NULL; pCopy = pCopy->next; } // third round recover original list for (std::map<RandomListNode *, RandomListNode *>::iterator i = oriNext.begin(); i != oriNext.end(); ++i) { i->first->next = i->second; } return dummy.next; } };
Tips:
典型的链表深拷贝,之前用python做的时候用了O(1)空间技巧(http://www.cnblogs.com/xbf9xbf/p/4216655.html)。
这次尝试了用hashmap的O(n)空间的技巧,原因是感觉hashmap可能更通用一些。
思路主要参考了下面这个日志:http://www.geeksforgeeks.org/a-linked-list-with-next-and-arbit-pointer/。
需要注意的是:如果原来的链表节点random为空,就不要再往下寻找next;需要加一个保护判断
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第二次过这道题,hashmap的大体思路画画图还记得,代码调试了几次才AC。
(1)不要忘记恢复原来的链表
(2)在拷贝random的时候,不要忘记copy = copy->next这条语句
/** * 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) return NULL; map<RandomListNode*, RandomListNode*> ori_next; RandomListNode* p = head; RandomListNode dummpy(-1); RandomListNode* copy = &dummpy; while (p) { copy->next = new RandomListNode(p->label); copy->next->random = p; ori_next[p] = p->next; p = p->next; copy->next->random->next = copy->next; copy = copy->next; } copy = dummpy.next; while (copy) { copy->random = copy->random->random ? copy->random->random->next : NULL; copy = copy->next; } for ( map<RandomListNode*, RandomListNode*>::iterator i=ori_next.begin(); i!=ori_next.end(); ++i ) { i->first->next = ori_next[i->first]; } return dummpy.next; } };