class Solution {

    List list = new LinkedList();

    public List<Integer> preorderTraversal(TreeNode root) {

        if (root == null) {
            return list;
        }

        list.add(root.val);
        preorderTraversal(root.left);
        preorderTraversal(root.right);

        return list;
    }
}

https://leetcode-cn.com/problems/binary-tree-preorder-traversal/