Given a binary tree, find its maximum depth.

The maximum depth is the number of nodes along the longest path from the root node down to the farthest leaf node.

public class Solution {
    public int maxDepth(TreeNode root) { 
        int depth = 0;
        if(root != null){
            int leftDepth = maxDepth(root.left);
            int rightDepth = maxDepth(root.right);
            depth ++;
            if(leftDepth < rightDepth){
                depth = depth + rightDepth;
            }else{
               depth = depth + leftDepth;
            }
        }

        return depth;

    } 

}