Day21: [LeetCode中等] 430. 扁平化多级双向链表

题源：

来自leetcode题库：

https://leetcode-cn.com/problems/flatten-a-multilevel-doubly-linked-list/

思路：

我用的是递归的思想，这道题很适合递归。

代码：

dirty code凑合看吧

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

    Node() {}

    Node(int _val, Node* _prev, Node* _next, Node* _child) {
        val = _val;
        prev = _prev;
        next = _next;
        child = _child;
    }
};
*/
Node *res,*rear;
void f(Node *head){
    auto p=head;
    while(p){
        Node* temp=new Node(p->val,NULL,NULL);
        if(rear){
            rear->next=temp;
            temp->prev=rear;
            temp->next=NULL;
            rear=rear->next;
        }else{
            rear=res=temp;
            temp->prev=NULL;
            temp->next=NULL;
        }
        if(p->child){
            f(p->child);
        }
        temp->child=NULL;
        p=p->next;
    }
}
class Solution {
public:
    Node* flatten(Node* head) {
        if(!head) return head;
        res=NULL;
        rear=NULL;
        f(head);
        return res;
    }
};