3
  9  20
   15   7

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 Math.max(left, right)+1;