leetcode - Binary Tree Preorder Traversal

Given a binary tree, return the preorder traversal of its nodes' values.

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

   1
    \
     2
    /
   3

return [1,2,3].

Note: Recursive solution is trivial, could you do it iteratively?

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */

//BinTree的先序遍歷
struct TreeNode
{
	int val;
	TreeNode *left;
	TreeNode *right;
	TreeNode(int x): val(x), left(NULL),right(NULL) {}
}
class Solution {
public:
	std::vector<int> preorderTraversal(TreeNode *root) {
		std::vector<int> vec;
		BinTree(root,vec);
		return vec;
	}
	void BinTree(TreeNode *root, std::vector<int> &vec)
	{
		if(root != NULL)
		{
			vec.push_back(root->vel);
			BinTree(root->left,vec);
			BinTree(root->right,vec);
		}
	}
};