leetcode - Binary Tree Postorder Traversal

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

For example:

Given binary tree

{1,#,2,3}

,

1
\
2
/
3

return

[3,2,1]

.

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的后序遍历
struct TreeNode
{
int val;
TreeNode *left;
TreeNode *right;
TreeNode(int x) : val(x),left(NULL),right(NULL) {}
};
class Solution {
public:
std::vector<int> postorderTraversal(TreeNode *root) {
std::vector<int> vec;
BinTree(root,vec);
return vec;
}
void BinTree(TreeNode *root,std::vector<int> &vec)
{
if(root != NULL)
{
BinTree(root->left,vec);
BinTree(root->right,vec);
vec.push_back(root->val);
}
}
private:
std::vector<int> vec;
};