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.
方法一:递归
时间复杂度:o(n) 空间复杂度:o(1)