Binary Tree Preorder Traversal

recursion

public class Solution {
    public ArrayList<Integer> preorderTraversal(TreeNode root) {
        // IMPORTANT: Please reset any member data you declared, as
        // the same Solution instance will be reused for each test case.
        ArrayList<Integer> result = new ArrayList<Integer>();
        traversal(result, root);
        return result;
    }
    private void traversal(ArrayList<Integer> result, TreeNode root)
    {
        if(root == null)
            return;
        result.add(root.val);
        traversal(result, root.left);
        traversal(result, root.right);
    }
}