class Solution {
    public List<Integer> preorderTraversal(TreeNode root) {
        List<Integer> list = new ArrayList<>();
        Stack<TreeNode> stack = new Stack<>();
        TreeNode cur = root;
        while(cur != null || !stack.isEmpty()){
            while(cur != null){
                stack.push(cur);
                list.add(cur.val);
                cur = cur.left;
            }
            TreeNode top = stack.pop();
            cur = top.right;
        }
        return list;
    }
}