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; } } ```