Merge pull request #646 from qxuewei/master

添加 142.环形链表II Swift版本

problems/0142.环形链表II.md

@@ -296,6 +296,39 @@ var detectCycle = function(head) {
 };
 ```
 
+Swift:
+```swift
+class Solution {
+    func detectCycle(_ head: ListNode?) -> ListNode? {
+        var slow: ListNode? = head
+        var fast: ListNode? = head
+        while fast != nil && fast?.next != nil {
+            slow = slow?.next
+            fast = fast?.next?.next
+            if slow == fast {
+                // 环内相遇
+                var list1: ListNode? = slow
+                var list2: ListNode? = head
+                while list1 != list2 {
+                    list1 = list1?.next
+                    list2 = list2?.next
+                }
+                return list2
+            }
+        }
+        return nil
+    }
+}
+extension ListNode: Equatable {
+    public func hash(into hasher: inout Hasher) {
+        hasher.combine(val)
+        hasher.combine(ObjectIdentifier(self))
+    }
+    public static func == (lhs: ListNode, rhs: ListNode) -> Bool {
+        return lhs === rhs
+    }
+}
+```
 
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 * 作者微信：[程序员Carl](https://mp.weixin.qq.com/s/b66DFkOp8OOxdZC_xLZxfw)