Binary Tree Postorder Traversal

Summary: Basic staff, need to check if goes up along the tree, that's the difference to pre-order traverse.

 1     vector<int> postorderTraversal(TreeNode *root) {

 2         vector<int> seq;

 3         if(root == NULL)

 4             return seq;

 5         stack<TreeNode *> nodes;

 6         nodes.push(root);

 7         TreeNode * pre_node = NULL;

 8         while(nodes.size() != 0) {

 9             TreeNode * node = nodes.top();

10             if(node->left == NULL && node->right == NULL || 

11                 (pre_node != NULL && (pre_node == node->left || pre_node == node->right))){

12                 pre_node = node;

13                 seq.push_back(node->val);

14                 nodes.pop();

15                 continue;

16             }

17             

18             if(node->right != NULL) {

19                 nodes.push(node->right);

20             }

21             

22             if(node->left != NULL)

23                 nodes.push(node->left);

24         }

25     }