feat: add swift implementation to lcci problem: No.02.08 (doocs#2630)

Author: klever34

lcci/02.08.Linked List Cycle/README.md

@@ -204,3 +204,37 @@ var detectCycle = function (head) {
     return null;
 };
 ```
+
+```swift
+/*
+* public class ListNode {
+*    var val: Int
+*    var next: ListNode?
+*    init(_ x: Int) {
+*        self.val = x
+*        self.next = nil
+*    }
+* }
+*/
+
+class Solution {
+    func detectCycle(_ head: ListNode?) -> ListNode? {
+        var slow = head
+        var fast = head
+
+        while fast != nil && fast?.next != nil {
+            slow = slow?.next
+            fast = fast?.next?.next
+            if slow === fast {
+                var ans = head
+                while ans !== slow {
+                    ans = ans?.next
+                    slow = slow?.next
+                }
+                return ans
+            }
+        }
+        return nil
+    }
+}
+```

lcci/02.08.Linked List Cycle/README_EN.md

@@ -233,6 +233,40 @@ var detectCycle = function (head) {
 };
 ```
 
+```swift
+/*
+* public class ListNode {
+*    var val: Int
+*    var next: ListNode?
+*    init(_ x: Int) {
+*        self.val = x
+*        self.next = nil
+*    }
+* }
+*/
+
+class Solution {
+    func detectCycle(_ head: ListNode?) -> ListNode? {
+        var slow = head
+        var fast = head
+
+        while fast != nil && fast?.next != nil {
+            slow = slow?.next
+            fast = fast?.next?.next
+            if slow === fast {
+                var ans = head
+                while ans !== slow {
+                    ans = ans?.next
+                    slow = slow?.next
+                }
+                return ans
+            }
+        }
+        return nil
+    }
+}
+```
+
 <!-- tabs:end -->
 
 <!-- end -->

lcci/02.08.Linked List Cycle/Solution.swift

@@ -0,0 +1,31 @@
+/*
+* public class ListNode {
+*    var val: Int
+*    var next: ListNode?
+*    init(_ x: Int) {
+*        self.val = x
+*        self.next = nil
+*    }
+* }
+*/
+
+class Solution {
+    func detectCycle(_ head: ListNode?) -> ListNode? {
+        var slow = head
+        var fast = head
+        
+        while fast != nil && fast?.next != nil {
+            slow = slow?.next
+            fast = fast?.next?.next
+            if slow === fast {
+                var ans = head
+                while ans !== slow {
+                    ans = ans?.next
+                    slow = slow?.next
+                }
+                return ans
+            }
+        }
+        return nil
+    }
+}