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;
        
    }
}