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的先序遍历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);}}};