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.
def max_depth(root) root ? [max_depth(root.left),max_depth(root.right)].max + 1 : 0 end
Leetcode 104 Maximum Depth of Binary Tree
原文：http://www.cnblogs.com/lilixu/p/4575119.html