144. Binary Tree Preorder Traversal

很簡單的前序走訪

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