diff --git a/src/BusStation.java b/src/BusStation.java
new file mode 100644
index 0000000..8823481
--- /dev/null
+++ b/src/BusStation.java
@@ -0,0 +1,45 @@
+public class BusStation {
+
+    private int num;
+    private int[] arr;
+    private int amount;
+
+    public BusStation(int num) {
+        this.num = num;
+        this.arr = new int[10];
+        this.amount = 1;
+    }
+
+    public boolean isStopping(int n){
+        for (int bus : arr) {
+            if (bus == n) {
+                return true;
+            }
+        }
+        return false;
+    }
+
+    public int getAmount() {
+        return amount;
+    }
+
+    public void setAmount(int amount) {
+        this.amount = amount;
+    }
+
+    public int getNum() {
+        return num;
+    }
+
+    public void setNum(int num) {
+        this.num = num;
+    }
+
+    public int[] getArr() {
+        return arr;
+    }
+
+    public void setArr(int[] arr) {
+        this.arr = arr;
+    }
+}
diff --git a/src/HelpCommands.java b/src/HelpCommands.java
index 9350cd2..98e91ea 100644
--- a/src/HelpCommands.java
+++ b/src/HelpCommands.java
@@ -621,6 +621,13 @@ public static Node<String> setFirst(Node<String> node, String value) {
         return ret;
     }
 
+    public static <T> void setLast(Node<T> node, T value) {
+        while (node.getNext() != null) {
+            node = node.getNext();
+        }
+        node.setNext(new Node<>(value));
+    }
+
     /**
      * @param fst the node to reverse
      * @return a reversed form of the given node
diff --git a/src/Item.java b/src/Item.java
new file mode 100644
index 0000000..9e25e85
--- /dev/null
+++ b/src/Item.java
@@ -0,0 +1,2 @@
+public record Item(int year, int score) {
+}
diff --git a/src/L21.java b/src/L21.java
new file mode 100644
index 0000000..32c8fef
--- /dev/null
+++ b/src/L21.java
@@ -0,0 +1,86 @@
+public class L21 extends HelpCommands {
+
+    public static boolean is3ConsecutiveNumbers(int n1, int n2, int n3) {
+        return (n2 - n1 == 1) && (n3 - n2 == 1);
+    }
+
+    public static boolean hasParticipatedIn3ConsecutiveYears(Runner runner) {
+        Node<Item> runnerItems = runner.lstItems();
+        while (runnerItems.getNext().getNext() != null) {
+            if (is3ConsecutiveNumbers(
+                    runnerItems.getValue().year(),
+                    runnerItems.getNext().getValue().year(),
+                    runnerItems.getNext().getNext().getValue().year()
+            )) {
+                return true;
+            }
+            runnerItems = runnerItems.getNext();
+        }
+        return false;
+    }
+
+    public static int numberOfRunnersParticipatedIn3ConsecutiveYears(MrtnRace mrtnRace) {
+        Node<Runner> runners = mrtnRace.lstRunners();
+        int numberOfRunnersParticipatedIn3ConsecutiveYears = 0;
+        while (runners != null) {
+            if (hasParticipatedIn3ConsecutiveYears(runners.getValue())) {
+                numberOfRunnersParticipatedIn3ConsecutiveYears++;
+            }
+            runners = runners.getNext();
+        }
+        return numberOfRunnersParticipatedIn3ConsecutiveYears;
+    }
+
+    public static Node<Integer> allStations(BusStation[] busStations, int[] autobusim){
+        Node<Integer> allStations = new Node<>(-1);
+        for (int autobus : autobusim){
+            boolean wasStopping = true;
+            boolean isStopping = true;
+            for (BusStation busStation : busStations){
+                if (wasStopping && busStation.isStopping(autobus)) {
+                    isStopping = true;
+                } else {
+                    wasStopping = false;
+                    isStopping = false;
+                }
+            }
+            if (wasStopping && isStopping) {
+                HelpCommands.setLast(allStations, autobus);
+            }
+        }
+        return allStations.getNext();
+    }
+
+    public static void main(String[] args) {
+//        Runner r1 = new Runner("1", 1);
+//        Runner r2 = new Runner("2", 2);
+//        Runner r3 = new Runner("3", 3);
+//
+//        r1.addItems(new Item(1, 2), new Item(2, 2), new Item(3, 3));
+//        r2.addItems(new Item(1, 2), new Item(5, 2), new Item(3, 3));
+//
+//        MrtnRace mrtnRace = new MrtnRace("Israel", 1);
+//        mrtnRace.addRunner(r1, r2);
+//
+//        mrtnRace.addScoreToRunner("1", 100);
+//        printNodes(r1.lstItems());
+//
+//        System.out.println(numberOfRunnersParticipatedIn3ConsecutiveYears(mrtnRace));
+
+        BusStation b1 = new BusStation(1);
+        b1.setArr(new int[]{49, 47});
+
+        BusStation b2 = new BusStation(2);
+        b2.setArr(new int[]{49, 47});
+
+        BusStation b3 = new BusStation(3);
+        b3.setArr(new int[]{49, 47});
+
+        BusStation[] busStations = new BusStation[]{b1, b2, b3};
+        int[] autobusim = new int[]{24, 47, 49, 42};
+
+        printNodes(allStations(busStations, autobusim));
+
+    }
+
+}
diff --git a/src/MrtnRace.java b/src/MrtnRace.java
new file mode 100644
index 0000000..4e7f80e
--- /dev/null
+++ b/src/MrtnRace.java
@@ -0,0 +1,42 @@
+public class MrtnRace {
+
+    public String country;
+    public int yearMrtn;
+    private Node<Runner> lstRunners;
+
+    public MrtnRace(String country, int yearMrtn) {
+        this.country = country;
+        this.yearMrtn = yearMrtn;
+        lstRunners = new Node<>(new Runner("-1", -1));
+    }
+
+    public Node<Runner> lstRunners(){
+        return lstRunners.getNext();
+    }
+
+    public void addRunner(Runner runner){
+        HelpCommands.setLast(lstRunners, runner);
+    }
+
+    public void addRunner(Runner... runners){
+        for (Runner runner : runners){
+            HelpCommands.setLast(lstRunners, runner);
+        }
+    }
+
+    public Node<Runner> findRunner(String id){
+        Node<Runner> temp = lstRunners;
+        while (temp != null) {
+            if (temp.getValue().id.equals(id)) {
+                return temp;
+            }
+            temp = temp.getNext();
+        }
+        return new Node<>(new Runner("-1", -1));
+    }
+
+    public void addScoreToRunner(String id, int score){
+        findRunner(id).getValue().addItem(new Item(yearMrtn, score));
+    }
+
+}
diff --git a/src/Runner.java b/src/Runner.java
new file mode 100644
index 0000000..af7589b
--- /dev/null
+++ b/src/Runner.java
@@ -0,0 +1,26 @@
+public class Runner {
+
+    public String id;
+    public int birthYear;
+    private Node<Item> lstItems;
+
+    public Runner(String id, int birthYear) {
+        this.id = id;
+        this.birthYear = birthYear;
+        lstItems = new Node<>(new Item(-1, -1));
+    }
+
+    public void addItem(Item item){
+        HelpCommands.setLast(lstItems, item);
+    }
+
+    public void addItems(Item... items){
+        for (Item item : items){
+            HelpCommands.setLast(lstItems, item);
+        }
+    }
+
+    public Node<Item> lstItems() {
+        return lstItems.getNext();
+    }
+}