From f51ffa7d3ca195803d4a62cc584c6f8f2d74da53 Mon Sep 17 00:00:00 2001
From: sowmya-rebbavarapu <sowmyarebbavarapu@gmail.com>
Date: Sat, 24 May 2025 12:16:59 +0530
Subject: [PATCH] Add linked list cycle detection tests and cycle creation
 utility

---
 .../linked-list/cycle/__tests__/cycle.test.js | 50 +++++++++++++++++++
 .../linked-list/cycle/createCycle.js          | 24 +++++++++
 .../linked-list/cycle/detectCycle.js          | 15 ++++++
 3 files changed, 89 insertions(+)
 create mode 100644 src/algorithms/linked-list/cycle/__tests__/cycle.test.js
 create mode 100644 src/algorithms/linked-list/cycle/createCycle.js
 create mode 100644 src/algorithms/linked-list/cycle/detectCycle.js

diff --git a/src/algorithms/linked-list/cycle/__tests__/cycle.test.js b/src/algorithms/linked-list/cycle/__tests__/cycle.test.js
new file mode 100644
index 0000000000..da76bc14c7
--- /dev/null
+++ b/src/algorithms/linked-list/cycle/__tests__/cycle.test.js
@@ -0,0 +1,50 @@
+import LinkedList from '../../../../data-structures/linked-list/LinkedList';
+import createCycle from '../createCycle';
+import detectCycle from '../detectCycle';
+
+describe('Linked List Cycle Tests', () => {
+  let list;
+
+  beforeEach(() => {
+    list = new LinkedList();
+  });
+
+  test('detectCycle returns false on list with no cycle', () => {
+    list
+      .append(1)
+      .append(9)
+      .append(87)
+      .append(5)
+      .append(22);
+    expect(detectCycle(list)).toBe(false);
+  });
+
+  test('createCycle creates a cycle and detectCycle detects it', () => {
+    list
+      .append(1)
+      .append(9)
+      .append(87)
+      .append(5)
+      .append(22);
+    createCycle(list, 2); // Creates cycle linking tail to node at index 2 (value 87)
+    expect(detectCycle(list)).toBe(true);
+  });
+
+  test('createCycle with invalid position does not create cycle', () => {
+    list
+      .append(1)
+      .append(9)
+      .append(87);
+    createCycle(list, 10); // Invalid index, no cycle should be created
+    expect(detectCycle(list)).toBe(false);
+  });
+
+  test('createCycle with position -1 does not create cycle', () => {
+    list
+      .append(1)
+      .append(9)
+      .append(87);
+    createCycle(list, -1); // No cycle created
+    expect(detectCycle(list)).toBe(false);
+  });
+});
diff --git a/src/algorithms/linked-list/cycle/createCycle.js b/src/algorithms/linked-list/cycle/createCycle.js
new file mode 100644
index 0000000000..88d43120db
--- /dev/null
+++ b/src/algorithms/linked-list/cycle/createCycle.js
@@ -0,0 +1,24 @@
+export default function createCycle(linkedList, position) {
+  if (!linkedList.head || position < 0) return;
+
+  let cycleNode = null;
+  let tail = linkedList.head;
+  let index = 0;
+
+  while (tail.next) {
+    if (index === position) {
+      cycleNode = tail;
+    }
+    tail = tail.next;
+    index += 1;
+  }
+
+  // For the last node
+  if (index === position) {
+    cycleNode = tail;
+  }
+
+  if (cycleNode) {
+    tail.next = cycleNode;
+  }
+}
diff --git a/src/algorithms/linked-list/cycle/detectCycle.js b/src/algorithms/linked-list/cycle/detectCycle.js
new file mode 100644
index 0000000000..e45a34729d
--- /dev/null
+++ b/src/algorithms/linked-list/cycle/detectCycle.js
@@ -0,0 +1,15 @@
+export default function detectCycle(linkedList) {
+  let slow = linkedList.head;
+  let fast = linkedList.head;
+
+  while (fast && fast.next) {
+    slow = slow.next;
+    fast = fast.next.next;
+
+    if (slow === fast) {
+      return true;
+    }
+  }
+
+  return false;
+}