feat: Add Floyd cycle-finding algorithm. (#9)

Author: goloroden

README.md

@@ -8,6 +8,7 @@ So far, the following exercises have been covered:
 
 - [Autocomplete](./autocomplete/) – implements an autocomplete feature using a trie
 - [Continous maximum](./continuousmax/) – calculates the maximum of a sliding window
+- [Floyd](./floyd/) – implements Floyd's cycle-finding "Tortoise and Hare" algorithm
 - [Heap](./heap/) – implements a heap from scratch, without using the built-in `container/heap` package
 - [Radix sort](./radixsort/) – implements radix sort
 - [Remove k-th last element](./removekthlastelement/) – removes the k-th last element from a single-linked list

floyd/documentation.go

@@ -0,0 +1,3 @@
+// Package floyd implements Floyd's cycle-finding "Tortoise and Hare"
+// algorithm.
+package floyd

floyd/has_cycle.go

@@ -0,0 +1,19 @@
+package floyd
+
+func HasCycle(head *Node) bool {
+	if head == nil {
+		return false
+	}
+
+	tortoise, hare := head, head
+	for hare != nil && hare.Next != nil {
+		tortoise = tortoise.Next
+		hare = hare.Next.Next
+
+		if tortoise == hare {
+			return true
+		}
+	}
+
+	return false
+}

floyd/has_cycle_test.go

@@ -0,0 +1,36 @@
+package floyd_test
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/thenativeweb/codingcircle/floyd"
+)
+
+func TestHasCycle(t *testing.T) {
+	t.Run("returns false for an empty list", func(t *testing.T) {
+		assert.False(t, floyd.HasCycle(nil))
+	})
+
+	t.Run("returns false for a single node", func(t *testing.T) {
+		list := floyd.NewList(2)
+
+		assert.False(t, floyd.HasCycle(list))
+	})
+
+	t.Run("returns false for a list without a cycle", func(t *testing.T) {
+		list := floyd.NewList(2, 3, 5, 7, 11)
+
+		assert.False(t, floyd.HasCycle(list))
+	})
+
+	t.Run("returns true for a list with a cycle", func(t *testing.T) {
+		list := floyd.NewList(2, 3, 5, 7, 11)
+
+		last := list.Next.Next.Next.Next
+		third := list.Next.Next
+		last.Next = third
+
+		assert.True(t, floyd.HasCycle(list))
+	})
+}

floyd/node.go

@@ -0,0 +1,22 @@
+package floyd
+
+type Node struct {
+	Value int
+	Next  *Node
+}
+
+func NewList(values ...int) *Node {
+	if len(values) == 0 {
+		return nil
+	}
+
+	head := &Node{values[0], nil}
+	current := head
+
+	for _, value := range values[1:] {
+		current.Next = &Node{value, nil}
+		current = current.Next
+	}
+
+	return head
+}