Preorder with stacks

class Solution {
public:
    void preorder(TreeNode * root, vector<int> &ans){

        if(root==NULL)return;

        stack<TreeNode*>s;

        s.push(root);

        while(s.size()!=0){
            TreeNode * temp = s.top();
            s.pop();
            ans.push_back(temp->val);

            if(temp->right!=NULL){
                s.push(temp->right);
            }
             if(temp->left!=NULL){
                s.push(temp->left);
            }

        }

        return;
    }

    vector<int> preorderTraversal(TreeNode* root) {

        vector<int> ans;
        preorder(root, ans);

        return ans;

    }
};