solves flatten multilevel doubly linked list

Author: anishLearnsToCode

README.md

@@ -286,6 +286,7 @@
 | 414  | [Third Maximum Number](https://leetcode.com/problems/third-maximum-number)                                                                                |                                     [![Java](assets/java.png)](src/ThirdMaximumNumber.java) [![Python](assets/python.png)](python/fizz_buzz.py)                                      |                                                                                                                                   |
 | 415  | [Add Strings](https://leetcode.com/problems/add-strings)                                                                                                  |                                         [![Java](assets/java.png)](src/AddString.java) [![Python](assets/python.png)](python/add_strings.py)                                         |                                                                                                                                   |
 | 422  | 🔒 [Valid Word Square](https://leetcode.com/problems/valid-word-square)                                                                                   |                                                                                                                                                                                      |                                                                                                                                   |
+| 430  | [Flatten a Multilevel Doubly Linked List](https://leetcode.com/problems/flatten-a-multilevel-doubly-linked-list)                                          |                                                       [![Java](assets/java.png)](src/FlattenAMultiLevelDoublyLinkedList.java)                                                        |                                                                                                                                   |
 | 434  | [Number of Segments in a String](https://leetcode.com/problems/number-of-segments-in-a-string)                                                            |                        [![Java](assets/java.png)](src/NumberOfSegmentsInString.java) [![Python](assets/python.png)](python/number_of_segments_in_a_string.py)                        |                                                                                                                                   |
 | 441  | [Arranging Coins](https://leetcode.com/problems/arranging-coins)                                                                                          |                                    [![Java](assets/java.png)](src/ArrangingCoins.java) [![Python](assets/python.png)](python/arranging_coins.py)                                     |                                                                                                                                   |
 | 443  | [String Compression](https://leetcode.com/problems/string-compression)                                                                                    |                                 [![Java](assets/java.png)](src/StringCompression.java) [![Python](assets/python.png)](python/string_compression.py)                                  |                                                                                                                                   |

src/FlattenAMultiLevelDoublyLinkedList.java

@@ -0,0 +1,49 @@
+// T: O(n)
+// S: O(n)
+
+public class FlattenAMultiLevelDoublyLinkedList {
+    private static final class Node {
+        public int val;
+        public Node prev;
+        public Node next;
+        public Node child;
+    }
+
+    public Node flatten(Node head) {
+        for (Node temp = head ; temp != null ; temp = temp.next) {
+            if (temp.child != null) {
+                Node lastNodeOfFlattenedChildren = flattenLinkedList(temp.child);
+                lastNodeOfFlattenedChildren.next = temp.next;
+                if (temp.next != null) {
+                    temp.next.prev = lastNodeOfFlattenedChildren;
+                }
+                temp.next = temp.child;
+                temp.child.prev = temp;
+                temp.child = null;
+            }
+        }
+        return head;
+    }
+
+    private Node flattenLinkedList(Node head) {
+        Node temp = head;
+        for (; temp.next != null ; temp = temp.next) {
+            if (temp.child != null) {
+                Node lastNodeOfFlattenedChildren = flattenLinkedList(temp.child);
+                lastNodeOfFlattenedChildren.next = temp.next;
+                temp.next.prev = lastNodeOfFlattenedChildren;
+                temp.next = temp.child;
+                temp.child.prev = temp;
+                temp.child = null;
+            }
+        }
+        if (temp.child != null) {
+            Node lastNodeOfFlattenedChildren = flattenLinkedList(temp.child);
+            temp.next = temp.child;
+            temp.child.prev = temp;
+            temp.child = null;
+            return lastNodeOfFlattenedChildren;
+        }
+        return temp;
+    }
+}