Given an N-ary tree, find the subtree with the maximum average. Return the root of the subtree.\nA subtree of a tree is the node which have **at least 1 child** plus all its descendants. The average value of a subtree is the sum of its values, divided by the number of nodes.\n\n**Example 1:**\n```\nInput:\n\t\t 20\n\t   /   \\\n\t 12     18\n  /  |  \\   / \\\n11   2   3 15  8\n\nOutput: 18\nExplanation:\nThere are 3 nodes which have children in this tree:\n12 => (11 + 2 + 3 + 12) / 4 = 7\n18 => (18 + 15 + 8) / 3 = 13.67\n20 => (12 + 11 + 2 + 3 + 18 + 15 + 8 + 20) / 8 = 11.125\n\n18 has the maximum average so output 18.\n```\n\nSimilar questions:\n* https://leetcode.com/problems/maximum-average-subtree

Given an N-ary tree, find the subtree with the maximum average. Return the root of the subtree.\nA subtree of a tree is the node which have at least 1 child plu