feat: Solve n-ary preorder lecture

  - Leetcode, #589

Author: brianr482

n-ary-tree-preorder-traversal.js

@@ -0,0 +1,35 @@
+/**
+ * Leetcode, 589
+ * Given an n-ary tree, return the preorder 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).
+ * 
+ * Follow up:
+ * Recursive solution is trivial, could you do it iteratively?
+ */
+
+
+/**
+ * // Definition for a Node.
+ * function Node(val,children) {
+ *    this.val = val;
+ *    this.children = children;
+ * };
+ */
+/**
+ * @param {Node} root
+ * @return {number[]}
+ */
+function preorder(root) {
+  const preOrder = [];
+  if (root) {
+    const stack = [root];
+    let node;
+    while (stack.length > 0) {
+      node = stack.pop();
+      preOrder.push(node.val);
+      stack.push(...node.children.reverse());
+    }
+  }
+  return preOrder;
+};