添加 0142.环形链表II.md Scala版本

problems/0142.环形链表II.md

@@ -370,7 +370,31 @@ ListNode *detectCycle(ListNode *head) {
 }
 ```
 
-
+Scala:
+```scala
+object Solution {
+  def detectCycle(head: ListNode): ListNode = {
+    var fast = head // 快指针
+    var slow = head // 慢指针
+    while (fast != null && fast.next != null) {
+      fast = fast.next.next // 快指针一次走两步
+      slow = slow.next // 慢指针一次走一步
+      // 如果相遇，fast快指针回到头
+      if (fast == slow) {
+        fast = head
+        // 两个指针一步一步的走，第一次相遇的节点必是入环节点
+        while (fast != slow) {
+          fast = fast.next
+          slow = slow.next
+        }
+        return fast
+      }
+    }
+    // 如果fast指向空值，必然无环返回null
+    null
+  }
+}
+```
 
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