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.

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public int maxDepth(TreeNode root) {
        //没有根节点
        if(root == null){
            return 0;
        }
        int dep1 = maxDepth(root.left);
        int dep2 = maxDepth(root.right);
        return dep1 > dep2 ? dep1+1:dep2+1;
    }
}