2019-12-29 430. Flatten a Multilevel Doubly Linked List Solutin

ruicore · ruicore · commit 0b6504a90242 · 2019-12-29T11:19:58.000+08:00
diff --git a/LeetCode/2019-12-29-430-Flatten-a-Multilevel-Doubly-Linked-List.md b/LeetCode/2019-12-29-430-Flatten-a-Multilevel-Doubly-Linked-List.md
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+# LeetCode 430. Flatten a Multilevel Doubly Linked List
+
+## Description
+
+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 1:
+
+```py
+Input: head = [1,2,3,4,5,6,null,null,null,7,8,9,10,null,null,11,12]
+Output: [1,2,3,7,8,11,12,9,10,4,5,6]
+Explanation:
+
+The multilevel linked list in the input is as follows:
+```
+
+
+After flattening the multilevel linked list it becomes:
+
+
+Example 2:
+```py
+Input: head = [1,2,null,3]
+Output: [1,3,2]
+Explanation:
+```
+```py
+The input multilevel linked list is as follows:
+
+  1---2---NULL
+  |
+  3---NULL
+```
+Example 3:
+
+```py
+Input: head = []
+Output: []
+```
+
+How multilevel linked list is represented in test case:
+
+```py
+We use the multilevel linked list from Example 1 above:
+
+ 1---2---3---4---5---6--NULL
+         |
+         7---8---9---10--NULL
+             |
+             11--12--NULL
+The serialization of each level is as follows:
+
+[1,2,3,4,5,6,null]
+[7,8,9,10,null]
+[11,12,null]
+To serialize all levels together we will add nulls in each level to signify no node connects to the upper node of the previous level. The serialization becomes:
+
+[1,2,3,4,5,6,null]
+[null,null,7,8,9,10,null]
+[null,11,12,null]
+Merging the serialization of each level and removing trailing nulls we obtain:
+
+[1,2,3,4,5,6,null,null,null,7,8,9,10,null,null,11,12]
+```
+
+### 思路
+
+* 使用循环，从头开始遍历，将子链看成一个整体，将整体向上移动一层。
+* 继续向后遍历，如果遇到子链，将其看作一层，将其整体向上移动，如此循环下去，直至结束。
+
+```py
+# -*- coding: utf-8 -*-
+# @Author:             何睿
+# @Create Date:        2019-12-29 10:57:03
+# @Last Modified by:   何睿
+# @Last Modified time: 2019-12-29 11:05:17
+
+
+class Node:
+    def __init__(self, val, prev, next, child):
+        self.val = val
+        self.prev = prev
+        self.next = next
+        self.child = child
+
+
+class Solution:
+    def flatten(self, head: 'Node') -> 'Node':
+        current = head
+        while current:
+            if current.child:
+                _next = current.next  # 当前节点的后一个节点
+                last = current.child  # 当前节点的自节点
+                while last.next:
+                    last = last.next  # 如果有子节点，找到子节点的最后一个节点
+                current.next = current.child
+                current.next.prev = current  # 将自节点向上提高一层
+                current.child = None
+                last.next = _next
+                if _next:
+                    _next.prev = last
+            current = current.next
+
+        return head
+```
+
+源代码文件在 [这里](https://github.com/ruicore/Algorithm/blob/master/LeetCode/2019-12-29-430-Flatten-a-Multilevel-Doubly-Linked-List.py) 。
+©本文首发于 何睿的博客 ，欢迎转载，转载需保留 [文章来源](https://ruicore.cn/leetcode-430-flatten-a-multilevel-doubly-linked-list/) ，作者信息和本声明.
