/*
knakul853
*/

class Solution {
public:
    Node* flatten(Node* head) {

       Node* cur = head;

        while(cur){

            if(!cur->child){
                cur = cur->next;
                continue;
            }
            else{

                Node* pre = cur->next;
                Node* temp = cur->child;
                while(temp->next)
                {
                    temp = temp->next;
                }

                temp->next = pre;
                if(pre) pre->prev = temp;
                cur->next = cur->child;

                cur->child->prev = cur;
                cur->child = NULL;

                cur = cur->next;
            }
        }

        return head;
    }
};