思考题：尝试使用迭代方式来解决一下。
Given a binary 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.
Note: A leaf is a node with no children.
Example:
Given binary tree [3,9,20,null,null,15,7],
3
/ \
9 20
/ \
15 7
return its depth = 3.
link:{sourcedir}/_0104_MaximumDepthOfBinaryTree.java[role=include]