# 430. Flatten a Multilevel Doubly Linked List

You are given a doubly linked list which in addition to the next and previous pointers, it could have a child pointer, which may or may not point to a separate doubly linked list. These child lists may have one or more children of their own, and so on, to produce a multilevel data structure, as shown in the example below.

Flatten the list so that all the nodes appear in a single-level, doubly linked list. You are given the head of the first level of the list.

Example:

``````Input:
1---2---3---4---5---6--NULL
|
7---8---9---10--NULL
|
11--12--NULL

Output:
1-2-3-7-8-11-12-9-10-4-5-6-NULL
``````

Explanation for the above example:

Given the following multilevel doubly linked list:

We should return the following flattened doubly linked list:

``````class Solution {
public:
while (cur) {
if (cur->child) {
Node *next = cur->next;
cur->child = flatten(cur->child);
Node *last = cur->child;
while (last->next) last = last->next;
cur->next = cur->child;
cur->next->prev = cur;
cur->child = NULL;
last->next = next;
if (next) next->prev = last;
}
cur = cur->next;
}
}
};
``````

``````class Solution {
public:
while (cur) {
if (cur->child) {
Node *next = cur->next;
Node *last = cur->child;
while (last->next) last = last->next;
cur->next = cur->child;
cur->next->prev = cur;
cur->child = NULL;
last->next = next;
if (next) next->prev = last;
}
cur = cur->next;
}
}
};
``````

Github 同步地址：

https://github.com/grandyang/leetcode/issues/430

Flatten Binary Tree to Linked List