Binary Tree Postorder Traversal

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

    vector<int> postorderTraversal(TreeNode *root) {
        vector<int> seq;
        if(root == NULL)
            return seq;
        stack<TreeNode *> nodes;
        nodes.push(root);
        TreeNode * pre_node = NULL;
        while(nodes.size() != 0) {
            TreeNode * node = nodes.top();
            if(node->left == NULL && node->right == NULL || 
                (pre_node != NULL && (pre_node == node->left || pre_node == node->right))){
                pre_node = node;
                seq.push_back(node->val);
                nodes.pop();
                continue;
            }
            
            if(node->right != NULL) {
                nodes.push(node->right);
            }
            
            if(node->left != NULL)
                nodes.push(node->left);
        }
    }