1.递归

  public List<Integer> preorderTraversal(TreeNode root) {
        List<Integer> res = new ArrayList<>();
        accessTree(root,res);
        return res;
    }

    public void accessTree(TreeNode root,List<Integer>res){
        if(root == null){
            return;
        }
        res.add(root.val);
        accessTree(root.left,res);

        accessTree(root.right,res);
    }

2.循环迭代

   public List<Integer> inorderTraversal(TreeNode root){
        List<Integer> res = new ArrayList<>();
        Deque<TreeNode> stack = new LinkedList<>();
        while (root!=null || !stack.isEmpty()){
            while (root!=null){
                stack.push(root);
                root = root.left;
            }
            root = stack.pop();
            res.add(root.val);
            root = root.right;

        }
        return res;
    }