Binary Tree Preorder Traversal

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