leetcode 145. Binary Tree Postorder Traversal

 1 vector<int> postorderTraversal(TreeNode *root) 
 2     {
 3         vector<int> tree_nodes;
 4         stack<TreeNode*> stree;
 5         TreeNode* visit = root, *peek = NULL, *last_visit = NULL;
 6         
 7         while (!stree.empty() || (visit != NULL))
 8         {
 9             if (visit != NULL)
10             {
11                 stree.push(visit);
12                 visit = visit->left;
13             }
14             else
15             {
16                 peek = stree.top();   
17                 if ((peek->right != last_visit) && (peek->right != NULL))
18                 {
19                     visit = peek->right;
20                 }
21                 else
22                 {
23                     tree_nodes.push_back(peek->val);
24                     last_visit = peek;
25                     stree.pop();
26                 }
27             }
28         }
29         
30         return tree_nodes;
31     }