138. Copy List with Random Pointer (Graph, Map; DFS)

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 *> flag;
        RandomListNode *root = copyNode(head, flag);
        return root;
    }

    RandomListNode * copyNode(RandomListNode *source, map<RandomListNode *, RandomListNode *> &flag)
    {
        if(flag.find(source)!=flag.end()) 
        {
            return flag[source];
        }

        RandomListNode *target = new RandomListNode (source->label);
        flag[source]=target;
        if(source->next)
            target->next = copyNode (source->next,flag);
        if(source->random)
            target->random = copyNode (source->random,flag);
        return target;
    }
};