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) {}
* };
*/
class Solution {
public:
vector<int> postorderTraversal(TreeNode *root)
{
vector<int> res;
stack<TreeNode* > sta;

TreeNode* cur = root;
TreeNode* pre = 0;
while (0 != cur || !sta.empty())
{
if (0 != cur)
{
sta.push(cur);
cur = cur->left;
}
else if (!sta.empty())
{
TreeNode* tmp = sta.top();
if (0 == tmp->right || tmp->right == pre)
{
res.push_back(tmp->val);
sta.pop();
pre = tmp;
}
else
cur = tmp->right;
}
}
return res;
}
};