Merge pull request #12 from programmingbookclub/meetup-21

21 20210223 linked list

Author: ytyubox

紀錄/21 20210223.md

@@ -0,0 +1,104 @@
+# LeetCode 讀書會第 21 次聚會 2021/02/23
+
+leetcode 讀書會通知
+1. 項目: 第 21  次聚會
+2. 目的: 線上一起寫題目, 由有想法的人帶領, 先解題, 再看該題有趣的解法
+3. 時間: 2/23 (二) 20:00 ~ 21:00
+4. 地點: google meet 線上 (前 10 分鐘預備鏈接)
+5. 解題項目:  [Linked list - ](https://leetcode.com/explore/learn/card/linked-list/210/doubly-linked-list/)
+
+    ```text
+  3. Medium    430. Flatten a Multilevel Doubly Linked 
+                                > List Hint: Try to solve this problem recursively.
+  4. Medium    708.  Insert into a Cyclic Sorted List 
+                                 >  Hint: A cyclic list here is a linked list that the last node is linked with the first node and form a cycle. Find out the smallest node will help you determine the suitable position.
+  5. Medium    138.  Copy List with Random Pointer 
+                                  > Hint: Using a hash set maybe is a good idea. Refer to our hash table card if you are not familiar with the hash set.
+    ```
+
+6. 共筆: GitHub https://github.com/programmingbookclub/Leetcode-club
+
+
+---
+
+
+```swift
+// Swift  Flatten a Multilevel Doubly Linked List
+
+
+/**
+ * Definition for a Node.
+ * public class Node {
+ *     public var val: Int
+ *     public var prev: Node?
+ *     public var next: Node?
+ *     public var child: Node?
+ *     public init(_ val: Int) {
+ *         self.val = val
+ *         self.prev = nil
+ *         self.next = nil
+ *         self.child  = nil
+ *     }
+ * }
+ */
+
+class Solution {
+    func flatten(_ head: Node?) -> Node? {
+        if head == nil {return head}
+        _ = dfs(head) 
+        return head
+    }
+    func dfs(_ head: Node?) -> Node? {
+        if head == nil {return nil}
+        var current = head
+       while true {
+           if current?.child != nil {
+               let childTail = dfs(current?.child)
+               childTail?.next = current?.next
+               current?.next?.prev = childTail
+               current?.next = current?.child 
+               current?.next?.prev = current
+               current?.child = nil
+           }
+           if current?.next == nil {
+               break
+           }
+           current = current?.next
+       } 
+        return current 
+    }
+}
+```
+
+
+```typescript=
+// Iterative in TS
+
+function flatten(head: Node | null): Node | null {
+    let curr = head;
+    let nextStack: Node[] = [];
+    while(curr instanceof Node) {
+        if (curr.child instanceof Node) {
+            if (curr.next instanceof Node) {
+                nextStack.push(curr.next);
+            }
+            linkNodes(curr, curr.child);
+            curr.child = null;
+            curr = curr.next;
+        }
+        else {
+            if (curr.next == null && nextStack.length > 0) {
+                linkNodes(curr, nextStack.pop());
+            }
+            curr = curr.next;
+        }
+    }
+    return head;
+};
+
+function linkNodes(prev: Node, next: Node): void {
+    prev.next = next;
+    next.prev = prev;
+}
+```
+