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) {}
 * };
 */
class Solution {
public:
    vector<int> postorderTraversal(TreeNode *root) {
        // IMPORTANT: Please reset any member data you declared, as
        // the same Solution instance will be reused for each test case.
        stack<TreeNode *> buffer;
        vector<int> output;
        if (root == NULL) return output;
        buffer.push(root);
        while (!buffer.empty()) {
            TreeNode *temp = buffer.top();
            if (temp->right == NULL && temp->left == NULL) {
                output.push_back(temp->val);
                buffer.pop();
                while (!buffer.empty() && 
                    (buffer.top()->left == temp || buffer.top()->right == temp)  ) {
                    temp = buffer.top();
                    output.push_back(temp->val);
                    buffer.pop();
                }
            } else {
                if (temp->right != NULL) 
                    buffer.push(temp->right);
                if (temp->left != NULL) 
                    buffer.push(temp->left);
            }
        }
        return output;
    }
};