Binary Tree Postorder Traversal

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:

class Solution {
public:
    void findNode(vector<int> &nodes,TreeNode *node){
        if(node->left)
            findNode(nodes,node->left);
        if(node->right)
            findNode(nodes,node->right);
        nodes.push_back(node->val);
        return;
    }
    
    vector<int> postorderTraversal(TreeNode *root) {
        vector<int> nodes;
        if(!root) return nodes;
        findNode(nodes,root);
        return nodes;
    }
};