题目：
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.
思路：
递归
package treetraversal; public class MaximumDepthOfBinaryTree { public int maxDepth(TreeNode root) { if (root == null) return 0; return Math.max(1 + maxDepth(root.left), 1 + maxDepth(root.right)); } }