Problem Binary Tree Preorder Traversal

Problem  Description:

Given a binary tree, return the preorder traversal of its nodes' values.

 Solution:

    public List<Integer> preorderTraversal(TreeNode root) {
        if (root == null) return new LinkedList<Integer>();
        List<Integer> list = new LinkedList<Integer>();
        if (root.left == null && root.right == null) {
            List<Integer> leaf = new LinkedList<Integer>();
            leaf.add(root.val);
            return leaf;
        } 
        list.add(root.val);
        if (root.left != null) list.addAll(preorderTraversal(root.left));
        if (root.right != null) list.addAll(preorderTraversal(root.right));
        
        return list;
    }