Add DepthFirstSearch with Java Streams (#2800)

Co-authored-by: xzero <andrejmezler@yahoo.de>

Searches/DepthFirstSearch.java

@@ -0,0 +1,90 @@
+package Searches;
+
+import java.util.ArrayList;
+import java.util.List;
+import java.util.Objects;
+import java.util.Optional;
+
+/**
+ * @author:  caos321
+ * @date:    31 October 2021 (Sunday)
+ */
+public class DepthFirstSearch {
+    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);
+        }
+    }
+}