class Solution {
   
  public List<Integer> preorderTraversal(TreeNode root) {
    List<Integer> result = new ArrayList<>();
    //处理边界情况
    if(root==null){
      return result;
    }
    //先访问根节点
    result.add(root.val);
    //再访问左子树
    List<Integer> leftChild = preorderTraversal(root.left);
    result.addAll(leftChild);
    //再访问右子树
    List<Integer> rightChild = preorderTraversal(root.right);
    result.addAll(rightChild);
    return result;
  }
}