leetcode - Path Sum II

Given a binary tree and a sum, find all root-to-leaf paths where each path's sum equals the given sum.

For example:
Given the below binary tree and sum = 22,

              5
             / \
            4   8
           /   / \
          11  13  4
         /  \    / \
        7    2  5   1

return

[
   [5,4,11,2],
   [5,8,4,5]
]

/**
 * 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<std::vector<int> > pathSum(TreeNode *root, int sum) {
        dfs(root,sum,0,std::vector<int>(0));
		return res;
    }
private:
	std::vector<std::vector<int>> res;
	void dfs(TreeNode *root, int sum, int n,std::vector<int> vec)
	{
		if(root == NULL) return;
		if(root->left == NULL && root->right == NULL)
		{
			if(sum == root->val + n)
			{
				vec.push_back(root->val);
				res.push_back(vec);
			}
		}
		else
		{
			vec.push_back(root->val);
			dfs(root->left,sum,n+root->val,vec);
			dfs(root->right,sum,n+root->val,vec);
		}
	}
};