430

/*
// Definition for a Node.
class Node {
public:
    int val;
    Node* prev;
    Node* next;
    Node* child;
};
*/

class Solution {
public:
    Node* flatten(Node* head) {
        for(Node *curr=head; curr; curr=curr->next){
            if(curr->child){
                Node *next = curr->next;
                Node *child = curr->child;
                child->prev = curr;
                curr->next = child;
                curr->child = NULL;
                while(child->next)
                    child = child->next;
                child->next = next;
                if(next)
                    next->prev = child;
            }
        }
        return head;
    };
};