Write a Program to Find N-ary Tree Postorder Traversal

# 590. N-ary Tree Postorder Traversal

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)

### Example 1:
```
Input: root = [1,null,3,2,4,null,5,6]
Output: [5,6,3,2,4,1]
```

### Example 2:
```
Input: root = [1,null,2,3,4,5,null,null,6,7,null,8,null,9,10,null,null,11,null,12,null,13,null,null,14]
Output: [2,6,14,11,7,3,12,8,4,13,9,10,5,1]
 ```

Constraints:
```
The number of nodes in the tree is in the range [0, 104].
0 <= Node.val <= 104
The height of the n-ary tree is less than or equal to 1000.
```

/**
 * // 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 out = [];
    const traverse = (node) => {
        if (!node) {
            return;
        }

        for (const item of node.children) {  
            traverse(item);
        }    
        out.push(node.val);    
        
    }
    traverse(root);
    return out;    
};