leetcode - Binary Tree Inorder Traversal

Given a binary tree, return the inorder traversal of its nodes‘ values.

For example:
Given binary tree {1,#,2,3},

   1
         2
    /
   3

return [1,3,2].

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
struct TreeNode
{
	int val;
	TreeNode *left;
	TreeNode *right;
	TreeNode(int x) : val(x), left(NULL), right(NULL) {}
};
class Solution {
public:
    std::vector<int> inorderTraversal(TreeNode *root) {
		dfs(root);
		return res;
    }
private:
	std::vector<int> res;
	void dfs(TreeNode *root)
	{
		if(root != NULL)
		{
			dfs(root->left);
			res.push_back(root->val);
			dfs(root->right);
		}
	}
};