Flatten a Multilevel Doubly Linked List problem solved

Author: hasibulislam999

Doubly-Linked List/430_flatten-a-multilevel-doubly-linked-list.js

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+/**
+ * Title: Flatten a Multilevel Doubly Linked List
+ * Description: You are given a doubly linked list, which contains nodes that have a next pointer, a previous pointer, and an additional child pointer. This child pointer may or may not point to a separate doubly linked list, also containing these special nodes. 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.
+ * Author: Hasibul Islam
+ * Date: 06/04/2023
+ */
+
+/**
+ * // Definition for a Node.
+ * function Node(val,prev,next,child) {
+ *    this.val = val;
+ *    this.prev = prev;
+ *    this.next = next;
+ *    this.child = child;
+ * };
+ */
+
+/**
+ * @param {Node} head
+ * @return {Node}
+ */
+
+// JavaScript Implementation of zhiying_qian's solution
+
+// Five situations:
+// 1. null - no need to flatten, just return it
+// 2. no child, no next - no need to flatten, it is the last element, just return it
+// 3. no child, next - no need to flatten, go next
+// 4. child, no next - flatten the child and done
+// 5. child, next - flatten the child, connect it with the next, go next
+
+var flatten = function (head) {
+  function traverse(node) {
+    if (node === null) return node; // CASE 1
+    if (node.child === null) {
+      if (node.next === null) return node; // CASE 2
+      else {
+        return traverse(node.next); // CASE 3
+      }
+    } else {
+      let child = node.child;
+      node.child = null;
+      let next = node.next;
+      node.next = child;
+      child.prev = node;
+      let childtail = traverse(child);
+      if (next !== null) {
+        // CASE 5
+        childtail.next = next;
+        next.prev = childtail;
+        return traverse(next);
+      }
+      return childtail;
+    }
+  }
+  traverse(head);
+  return head;
+};
+
+var Node = function (val, prev, next, child) {
+  this.val = val;
+  this.prev = prev;
+  this.next = next;
+  this.child = child;
+
+  this.child = null;
+  this.next = null;
+  this.prev = null;
+};
+
+console.log(
+  flatten(
+    new Node(
+      1,
+      null,
+      null,
+      new Node(
+        2,
+        null,
+        null,
+        new Node(
+          3,
+          null,
+          null,
+          new Node(4, null, null, new Node(5, null, null, null))
+        )
+      )
+    )
+  )
+);