题目编号：104
二叉树的最大深度

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public int maxDepth(TreeNode root) {
        if(root == null) {
            return 0;
        } else {
            int left = maxDepth(root.left);
            int right = maxDepth(root.right);
            return Math.max(left, right) + 1;
        }
    }
}