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Commit 53801df

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Merge pull request SharingSource#153 from SharingSource/ac_oier
✨update: Modify 430
2 parents 5ef67ec + b620969 commit 53801df

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‎LeetCode/421-430/430. 扁平化多级双向链表(中等).md‎

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```
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* 时间复杂度:最坏情况下,每个节点会被访问 $h$ 次($h$ 为递归深度,最坏情况下 $h = n$)。整体复杂度为 $O(n^2)$
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* 空间复杂度:最坏情况下所有节点都分布在 `child` 中,此时递归深度为 $n$。复杂度为 $O(n)$
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---
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### 递归(优化)
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在上述解法中,由于我们直接使用 `flatten` 作为递归函数,导致递归处理 $head.child$ 后不得不再进行遍历来找当前层的"尾结点",这导致算法复杂度为 $O(n^2)$。
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一个可行的优化是,额外设计一个递归函数 `dfs` 用于返回扁平化后的链表"尾结点",从而确保我们找尾结点的动作不会在每层发生。
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![image.png](https://pic.leetcode-cn.com/1632439410-oXoxbn-image.png)
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代码:
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```Java
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class Solution {
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public Node flatten(Node head) {
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dfs(head);
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return head;
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}
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Node dfs(Node head) {
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Node last = head;
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while (head != null) {
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if (head.child == null) {
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last = head;
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head = head.next;
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} else {
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Node tmp = head.next;
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Node childLast = dfs(head.child);
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head.next = head.child;
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head.child.prev = head;
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head.child = null;
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if (childLast != null) childLast.next = tmp;
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if (tmp != null) tmp.prev = childLast;
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last = head;
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head = childLast;
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}
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}
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return last;
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}
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}
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```
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* 时间复杂度:$O(n)$
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* 空间复杂度:最坏情况下所有节点都分布在 `child` 中,此时递归深度为 $n$。复杂度为 $O(n)$
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