[Hacker Rank] Interview Preparation Kit: Linked Lists: Detect a Cycle. Solved ✓.

Author: Gonzalo Diaz

algorithm-exercises-csharp-test/src/hackerrank/interview_preparation_kit/linked_list/LinkedListCycle.Test.cs

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+namespace algorithm_exercises_csharp.hackerrank.interview_preparation_kit;
+
+[TestClass]
+public class LinkedListCycleTest
+{
+  class LinkedListCycleTestCase
+  {
+    public string title = "";
+    public LinkedList.Node? llist;
+    public bool expected;
+  }
+
+  private static LinkedListCycleTestCase[] tests = [];
+
+  [TestInitialize]
+  public void testInitialize()
+  {
+    // Linked list sample data:
+    LinkedList.Node ll1_1 = new(1);
+    LinkedList.Node ll1_2 = new(2);
+    LinkedList.Node ll1_3 = new(3);
+    LinkedList.Node ll1_4 = new(4);
+    LinkedList.Node ll1_5 = new(5);
+
+    ll1_1.next = ll1_2;
+    ll1_2.next = ll1_3;
+    ll1_3.next = ll1_4;
+    ll1_4.next = ll1_5;
+    ll1_4.next = ll1_3; // <- cycle
+
+    LinkedList.Node ll2_1 = new(1);
+    LinkedList.Node ll2_2 = new(2);
+    LinkedList.Node ll2_3 = new(3);
+
+    ll2_1.next = ll2_2;
+    ll2_2.next = ll2_3;
+
+    tests = [
+      new()
+      {
+        title = "Sample Test case X",
+        llist = ll1_1,
+        expected = true
+      },
+      new()
+      {
+        title = "Sample Test case Y",
+        llist = ll2_1,
+        expected = false
+      }
+    ];
+  }
+
+  [TestMethod]
+  public void testLinkedListCycle()
+  {
+    bool result;
+
+    foreach (LinkedListCycleTestCase test in tests)
+    {
+      result = LinkedListCycle.hasCycle(test.llist);
+      Assert.AreEqual(
+        test.expected,
+        result,
+        String.Format(
+          "{0} testLinkedListCycle({1}) => must be: {2}",
+          test.title,
+          test.llist,
+          test.expected
+        )
+      );
+    }
+  }
+}

algorithm-exercises-csharp/src/hackerrank/interview_preparation_kit/linked_list/LinkedListCycle.cs

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+// @link Problem definition [[docs/hackerrank/interview_preparation_kit/linked_lists/ctci_linked_list_cycle.md]]
+
+namespace algorithm_exercises_csharp.hackerrank.interview_preparation_kit;
+
+using System.Diagnostics.CodeAnalysis;
+
+public class LinkedListCycle
+{
+  [ExcludeFromCodeCoverage]
+  protected LinkedListCycle() { }
+
+  public static bool hasCycle(LinkedList.Node? head)
+  {
+    List<LinkedList.Node> llindex = [];
+
+    LinkedList.Node? node = head;
+
+    while (node != null)
+    {
+      if (llindex.Contains(node))
+      {
+        return true;
+      }
+
+      llindex.Add(node);
+      node = node.next;
+    }
+
+    return false;
+  }
+}

docs/hackerrank/interview_preparation_kit/linked_lists/ctci_linked_list_cycle.md

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+# [Linked Lists: Detect a Cycle](https://www.hackerrank.com/challenges/ctci-linked-list-cycle)
+
+- Difficulty:  `#easy`
+- Category: `#ProblemSolvingIntermediate`
+
+A linked list is said to contain a cycle if any node is visited more
+than once while traversing the list.
+For example, in the following graph there is a cycle formed
+when node `5` points back to node `3`.
+
+```mermaid
+flowchart LR
+
+      1([1])
+      2([2])
+      3([3])
+      4([4])
+      5([5])
+
+      1 --> 2
+      2 --> 3
+      3 --> 4
+      4 --> 5
+      5 --> 3
+
+    classDef default color:#000,fill:#97FE9C,stroke:#488A53,stroke-width:4px;
+```
+
+## Function Description
+
+Complete the function has_cycle in the editor below.
+It must return a boolean true if the graph contains a cycle, or false.
+
+has_cycle has the following parameter(s):
+
+- head: a pointer to a Node object that points to the head of a linked list.
+
+## Returns
+
+- boolean: True if there is a cycle, False if there is not
+
+**Note**: If the list is empty, `head` will be null.
+
+## Input Format
+
+There is no input for this challenge.
+A random linked list is generated at runtime and passed to your function.
+
+## Constraints
+
+- $ 0 \leq \text{list size} \leq 100$
+
+## Sample Input
+
+The following linked lists are passed as arguments to your function:
+
+```mermaid
+flowchart LR
+
+      1([1])
+      1 --> NULL
+
+    classDef default color:#000,fill:#97FE9C,stroke:#488A53,stroke-width:4px;
+
+    classDef none color: #DDD, fill:#FFFFFF00,fill:#FFFFFF00, stroke:#FFFFFF00, stroke-width: 0;
+
+    class NULL none
+```
+
+```mermaid
+flowchart LR
+
+      1([1])
+      2([2])
+      3([3])
+
+      1 --> 2
+      2 --> 3
+      3 --> 2
+
+    classDef default color:#000,fill:#97FE9C,stroke:#488A53,stroke-width:4px;
+
+    classDef none color: #DDD, fill:#FFFFFF00,fill:#FFFFFF00, stroke:#FFFFFF00, stroke-width: 0;
+
+    class NULL none
+```
+
+## Sample Output
+
+```text
+0
+1
+```
+
+## Explanation
+
+1. The first list has no cycle, so we return false and the
+hidden code checker prints 0 to stdout.
+2. The second list has a cycle, so we return true and the
+hidden code checker prints 1 to stdout.