Day2：二叉树的最大深度

leetcode链接：https://leetcode-cn.com/problems/maximum-depth-of-binary-tree/

递归的做法：

**
 * 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;
        int left = maxDepth(root.left);
        int right = maxDepth(root.right);
        return (left > right)?left+1:right+1;