把中序遍历的代码copy过来，直接改：
public class Solution {
public List<Integer> preorderTraversal(TreeNode root) {
List<Integer> res = new ArrayList<>();
helper(root, res);
return res;
}
private void helper(TreeNode root, List<Integer> res) {
if (root != null) {
res.add(root.val);
if (root.left != null) helper(root.left, res);
if (root.right != null) helper(root.right, res);
}
}
}