144. Binary Tree Preorder Traversal.cpp

class Solution {
public:
    void helper(TreeNode* root, vector<int>& ans){
        if(root == nullptr){
            return;
        }

        ans.push_back(root->val);
        helper(root->left, ans);
        helper(root->right, ans);
    }

    vector<int> preorderTraversal(TreeNode* root) {
        vector<int> ans;
        helper(root, ans);
        return ans;
    }
};