Update linked_list.md

add get mid node and weather has a circle

Author: lxqqxl

zh-hans/basics_data_structure/linked_list.md

@@ -218,3 +218,39 @@ class NodeCircle:
         else:
             return False
 ```
+```c++: is a circle
+class List
+{
+public:
+bool iscircle(ListNode* head)
+{
+	if (!head || !head->next) return false;
+	ListNode *low = head, *fast = high;
+	while(fast && fast->next)
+	{
+		low = low->next;
+		fast = fast->next->next;
+		if (low == fast) return true;
+	}
+	return false;
+}
+};
+```
+
+```c++
+class List
+{
+public:
+ListNode* InLisNodet(ListNode *head)
+{
+	if (!head || !head->next) return head;
+	ListNode *low = head, *fast = head;
+	while(fast && fast->next)
+	{
+		low = low->next;
+		fast = fast->next->next;
+	}
+	return low;
+}
+};
+```