Grokking 231

class Solution {
    public Node findRoot(List<Node> tree) {
        Set<Node> childNodes = new HashSet<>();

        for(Node node : tree) {
            for (Node childNode : node.children) {
                childNodes.add(childNode);
            }
        }

        for(Node node : tree) {
            if (!childNodes.contains(node)) {
                return node;
            }
        }

        return null;
    }
}