题目：

题解：

class Solution {
    public int maxDepth(Node root) {
        if (root == null) {
            return 0;
        }
        Queue<Node> queue = new LinkedList<Node>();
        queue.offer(root);
        int ans = 0;
        while (!queue.isEmpty()) {
            int size = queue.size();
            while (size > 0) {
                Node node = queue.poll();
                List<Node> children = node.children;
                for (Node child : children) {
                    queue.offer(child);
                }
                size--;
            }
            ans++;
        }
        return ans;
    }
}