Binary Tree Postorder Traversal

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

For example:
Given binary tree [1,null,2,3],

1
\
2
/
3

return [3,2,1].

Note: you do it iteratively.

/**
 * Definition for a binary tree node.
 * 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> result;
        std::unordered_map<TreeNode *, bool> visited_map;
        std::stack<TreeNode *> node_stack;
        TreeNode * current = root;
        while(current || !node_stack.empty())
        {
            while(current)
            {
                node_stack.push(current);
                visited_map[current] = false;
                current = current->left;
            }
            TreeNode * top_node = node_stack.top();
            if(visited_map[top_node])
            {
                result.push_back(top_node->val);
                node_stack.pop();
            }
            else
            {
                visited_map[top_node] = true;
                current = top_node->right;
            }
        }
        return result;
    }
};