Add solution #590

Author: JoshCrozier

README.md

@@ -457,6 +457,7 @@
 583|[Delete Operation for Two Strings](./0583-delete-operation-for-two-strings.js)|Medium|
 587|[Erect the Fence](./0587-erect-the-fence.js)|Hard|
 589|[N-ary Tree Preorder Traversal](./0589-n-ary-tree-preorder-traversal.js)|Easy|
+590|[N-ary Tree Postorder Traversal](./0590-n-ary-tree-postorder-traversal.js)|Easy|
 594|[Longest Harmonious Subsequence](./0594-longest-harmonious-subsequence.js)|Easy|
 599|[Minimum Index Sum of Two Lists](./0599-minimum-index-sum-of-two-lists.js)|Easy|
 605|[Can Place Flowers](./0605-can-place-flowers.js)|Easy|

solutions/0590-n-ary-tree-postorder-traversal.js

@@ -0,0 +1,34 @@
+/**
+ * 590. N-ary Tree Postorder Traversal
+ * https://leetcode.com/problems/n-ary-tree-postorder-traversal/
+ * Difficulty: Easy
+ *
+ * Given the root of an n-ary tree, return the postorder traversal of its nodes' values.
+ *
+ * Nary-Tree input serialization is represented in their level order traversal. Each group
+ * of children is separated by the null value (See examples)
+ */
+
+/**
+ * // Definition for a _Node.
+ * function _Node(val,children) {
+ *    this.val = val;
+ *    this.children = children;
+ * };
+ */
+
+/**
+ * @param {_Node|null} root
+ * @return {number[]}
+ */
+var postorder = function(root) {
+  const result = [];
+  traverse(root);
+  return result;
+
+  function traverse(node) {
+    if (!node) return;
+    node.children.forEach(child => traverse(child));
+    result.push(node.val);
+  }
+};