秒杀
/**

 * Definition for binary tree

 * public class TreeNode {

 *     int val;

 *     TreeNode left;

 *     TreeNode right;

 *     TreeNode(int x) { val = x; }

 * }

 */

public class Solution {

    public int maxDepth(TreeNode root) {

        if(root==null) return 0;

      int h1=maxDepth(root.left);

      int h2=maxDepth(root.right);

      return h1>h2?h1+1:h2+1;

    }

}