DepthFirstSearch.java

import java.util.ArrayList;
import java.util.List;
import java.util.Objects;
import java.util.Optional;

/**
 * author:  caos321
 * date:    20 October 2022 (Thursday)
 */
public class DepthFirstSearch {

    private static class Node {
        private final String name;
        private final List<Node> subNodes;

        public Node(final String name) {
            this.name = name;
            this.subNodes = new ArrayList<>();
        }

        public Node(final String name, final List<Node> subNodes) {
            this.name = name;
            this.subNodes = subNodes;
        }

        public String getName() {
            return name;
        }

        public List<Node> getSubNodes() {
            return subNodes;
        }
    }

    public static Optional<Node> search(final Node node, final String name) {
        if (node.getName().equals(name)) {
            return Optional.of(node);
        }

        return node.getSubNodes()
                .stream()
                .map(value -> search(value, name))
                .flatMap(Optional::stream)
                .findAny();
    }

    public static void assertThat(final Object actual, final Object expected) {
        if (!Objects.equals(actual, expected)) {
            throw new AssertionError(String.format("expected=%s but was actual=%s", expected, actual));
        }
    }

    public static void main(final String[] args) {
        final Node rootNode = new Node("A", List.of(
                new Node("B", List.of(new Node("D"), new Node("F", List.of(
                        new Node("H"), new Node("I")
                )))),
                new Node("C", List.of(new Node("G"))),
                new Node("E")
        ));

        {
            final String expected = "I";

            final Node result = search(rootNode, expected)
                    .orElseThrow(() -> new AssertionError("Node not found!"));

            assertThat(result.getName(), expected);
        }

        {
            final String expected = "G";

            final Node result = search(rootNode, expected)
                    .orElseThrow(() -> new AssertionError("Node not found!"));

            assertThat(result.getName(), expected);
        }

        {
            final String expected = "E";

            final Node result = search(rootNode, expected)
                    .orElseThrow(() -> new AssertionError("Node not found!"));

            assertThat(result.getName(), expected);
        }
    }
}