Given a n-ary tree, find its maximum depth.

The maximum depth is the number of nodes along the longest path from the root node down to the farthest leaf node.

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: 3

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: 5

Constraints:

- The total number of nodes is in the range `[0, 104]`.
- The depth of the n-ary tree is less than or equal to `1000`.