递归比较左右子树即可。
import java.util.*;
public class Solution {
public int maxDepth (TreeNode root) {
if(root==null)return 0;
if(root.left==null&&root.right==null)return 1;
int leftDepth = maxDepth(root.left)+1;
int rightDepth = maxDepth(root.right)+1;
return leftDepth>rightDepth?leftDepth:rightDepth;
}
}