Given the root of a binary tree, return its maximum depth.
A binary tree's maximum depth is the number of nodes along the longest path from the root node down to the farthest leaf node.
Example 1:
Input: root = [3,9,20,null,null,15,7]
Output: 3
Example 2:
Input: root = [1,null,2]
Output: 2
Constraints:
[$0, 10^4$].100 <= Node.val <= 100給一個二元樹的根結點 root
要找出二元樹的最大深度
每個結點都是由左結點跟右結點所組成
所以對於每個結點其最大深度 = Max(左結點最大深度，右結點最大深度) + 1(因為加上目前這層深度)
當該結點是空值時，最大深度定義是 0(因為不存在)
https://replit.com/@YuanyuLiang/golangmaxtreedepth#sol/solution.go