Maximum Depth of Binary Tree

description:
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.

Have you met this question in a real interview? Yes
Example
Given a binary tree as follow:

1
/ \
2 3
/ \
4 5
The maximum depth is 3.
bug free
解法：使用DFS的方式进行计算 ==> 每当遇到二叉树的问题，想想整个树在该问题上的结果与树的左右儿子之间的关系的联系。
在使用recursion的方式计算时，没有必要想真个树的流程，只要想清楚一层之间的关系即可。

/**
 * Definition of TreeNode:
 * public class TreeNode {
 *     public int val;
 *     public TreeNode left, right;
 *     public TreeNode(int val) {
 *         this.val = val;
 *         this.left = this.right = null;
 *     }
 * }
 */
public class Solution {
    /**
     * @param root: The root of binary tree.
     * @return: An integer.
     */
    public int maxDepth(TreeNode root) {
        // write your code here
        int count = 0;
        if (root == null) {
            return count;
        }
        int left = maxDepth(root.left);
        int right = maxDepth(root.right);
        return Math.max(left, right) + 1;
    }
}