leetcode430.cpp

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

class Solution {
public:
    Node* flatten(Node* head) {
        
        if(head == NULL){
            return head;
        }

        Node* curr = head;

        while(curr!=NULL){

            if(curr->child != NULL){
                 
                 Node* next = curr->next;

                 curr->next =flatten(curr->child);


                 curr->next->prev = curr;

                 curr->child = NULL;


                 while(curr->next != NULL){
                    curr=curr->next;

                    
                 }
                 if(next != NULL){
                    curr->next =next;

                    next->prev= curr;
                 }



            }
            curr=curr->next;
        }
        return head;
    }
};