import java.util.*;

/*
 * public class TreeNode {
 *   int val = 0;
 *   TreeNode left = null;
 *   TreeNode right = null;
 * }
 */

public class Solution {
    /**
     * 
     * @param root TreeNode类 
     * @return int整型
     */
    public int maxDepth (TreeNode root) {
        // write code here
        if(root==null) return 0;
        int left = 0, right=0;
        if(root.left!=null) left = maxDepth(root.left);
        if(root.right!=null) right = maxDepth(root.right);
        return Math.max(left,right)+1;
    }
}