class Solution {

    public int maxDepth(TreeNode root) {

        if (root == null){
            return 0;
        }

        int leftDepth = maxDepth(root.left);
        int rightDepth = maxDepth(root.right);

        return Math.max(leftDepth, rightDepth) + 1;
    }
}

https://leetcode-cn.com/problems/maximum-depth-of-binary-tree/