LeetCode Problem : Binary Tree Postorder Traversal

Problem 

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

For example:
Given binary tree {1,#,2,3},
   1
    \
     2
    /
   3
return [3,2,1].

Note: Recursive solution is trivial, could you do it iteratively?

Code

vector<int> postorderTraversal(TreeNode *root) {
    // IMPORTANT: Please reset any member data you declared, as
    // the same Solution instance will be reused for each test case.
    vector<int> result;
    if(!root)
        return result; 
    stack<TreeNode *> st;
    do{
        while(root){
            if(root->right)
                st.push(root->right);
            st.push(root);
            root = root->left;
        }
        TreeNode *node = st.top();
        st.pop();
        if(node->right && !st.empty() && node->right == st.top()){
            root = st.top();
            st.pop();
            st.push(node);
        }
        else{
            result.push_back(node->val);
            root = 0;
        }
    }while(!st.empty());
    return result;
}