Binary Tree Traversal without Recursion 1

  
 vector<int> preorderTraversal(TreeNode *root) {
        stack<TreeNode*> st;
        vector<int> ret;
        if (root==NULL)
        return ret;
        TreeNode *r = root;
       
        while(r!=NULL||!st.empty())
        {
            while (r!=NULL)
            {
                st.push(r);
                ret.push_back(r->val);
                r=r->left;
            }
            r = st.top();
            st.pop();
            r=r->right;
        }
       
        return ret;
    }

    vector<int> inorderTraversal(TreeNode *root) {
        stack<TreeNode *> st;
        vector<int> ret;
        if (root==NULL)
        return ret;
        TreeNode *r = root;
       
        while(r||!st.empty())
        {
            while(r)
            {
                st.push(r);
                r = r->left;
            }
            TreeNode *tmp = st.top();
            st.pop();
            ret.push_back(tmp->val);
            r = tmp->right;
        }
       
        return ret;
    }

    vector<int> postorderTraversal(TreeNode *root) {
        vector<int> ret;
        if (root==NULL)
        return ret;
        stack<TreeNode *> st;
        TreeNode *r = root;
        TreeNode *lastVisitedNode=NULL;
        while(!st.empty()||r)
        {
            while(r)
            {
                st.push(r);
                r=r->left;
            }
           
            TreeNode *tmp = st.top();
            if (lastVisitedNode!=tmp->right&&tmp->right)
            r=tmp->right;
            else
            {
                st.pop();
                ret.push_back(tmp->val);
                lastVisitedNode = tmp;
            }
        }
        return ret;
    }