104. Maximum Depth of Binary Tree

做题历程：

1. 应该做了不止2次了，本次大概耗时6分钟，独立解出

本题相当基础，就是用dfs把全部点都遍历一遍，这次用的是recursive dfs，下次如果再做这道题，要挑战iterative dfs 代码如下：

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    int maxDepth(TreeNode* root) {
        longest = 0;
        dfs(root, 0);
        return longest;
    }
private:
    int longest;
    void dfs(TreeNode* node, int depth) {
        if (node == nullptr) {
            longest = depth > longest ? depth : longest;
        }
        else {
            ++depth;
            dfs(node->left, depth);
            dfs(node->right, depth);
        }
    }
};