From 33e2f1aa4c2930b5458bee6fefd528a6b7dee2a7 Mon Sep 17 00:00:00 2001
From: huangge1199 <huangge1199@hotmail.com>
Date: Fri, 19 Feb 2021 10:13:08 +0800
Subject: [PATCH] =?UTF-8?q?=E5=8A=9B=E6=89=A3=EF=BC=9A430.=20=E6=89=81?=
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---
 .../code/leet/Official/t20210219/Flatten.java | 50 +++++++++++++++++++
 1 file changed, 50 insertions(+)
 create mode 100644 LeetCode/src/main/java/com/code/leet/Official/t20210219/Flatten.java

diff --git a/LeetCode/src/main/java/com/code/leet/Official/t20210219/Flatten.java b/LeetCode/src/main/java/com/code/leet/Official/t20210219/Flatten.java
new file mode 100644
index 0000000..6822076
--- /dev/null
+++ b/LeetCode/src/main/java/com/code/leet/Official/t20210219/Flatten.java
@@ -0,0 +1,50 @@
+package com.code.leet.Official.t20210219;
+
+import com.code.leet.entiy.Node;
+
+/**
+ * 多级双向链表中，除了指向下一个节点和前一个节点指针之外，它还有一个子链表指针，可能指向单独的双向链表。这些子列表也可能会有一个或多个自己的子项，依此类推，生成多级数据结构，如下面的示例所示。
+ * <p>
+ * 给你位于列表第一级的头节点，请你扁平化列表，使所有结点出现在单级双链表中。
+ * <p>
+ *  
+ * <p>
+ * 示例 1：
+ * <p>
+ * 输入：head = [1,2,3,4,5,6,null,null,null,7,8,9,10,null,null,11,12]
+ * 输出：[1,2,3,7,8,11,12,9,10,4,5,6]
+ * <p>
+ * 来源：力扣（LeetCode）
+ * 链接：https://leetcode-cn.com/problems/flatten-a-multilevel-doubly-linked-list
+ * 著作权归领扣网络所有。商业转载请联系官方授权，非商业转载请注明出处。
+ */
+public class Flatten {
+    /**
+     * 430. 扁平化多级双向链表
+     */
+    public Node flatten(Node head) {
+        if (head == null) return head;
+        // pseudo head to ensure the `prev` pointer is never none
+        Node pseudoHead = new Node(0, null, head, null);
+
+        flattenDFS(pseudoHead, head);
+
+        // detach the pseudo head from the real head
+        pseudoHead.next.prev = null;
+        return pseudoHead.next;
+    }
+    /* return the tail of the flatten list */
+    public Node flattenDFS(Node prev, Node curr) {
+        if (curr == null) return prev;
+        curr.prev = prev;
+        prev.next = curr;
+
+        // the curr.next would be tempered in the recursive function
+        Node tempNext = curr.next;
+
+        Node tail = flattenDFS(curr, curr.child);
+        curr.child = null;
+
+        return flattenDFS(tail, tempNext);
+    }
+}