一快一慢两个指针，在存在环的链表里面的数学信息

Author: elfxwt

List/Linked List CycleI

@@ -33,3 +33,30 @@
 
 	  }
   };
+  //  数学
+    class Solution {
+  public:
+	  ListNode *detectCycle(ListNode *head) {
+		  ListNode *slow = head,*fast = head;
+		  while (fast && fast->next) // 防止空指针异常
+		  {
+			  slow = slow ->next;
+			  fast = fast->next->next;
+			  if(slow == fast)
+				  break;
+		  }
+		  if(fast==nullptr || fast->next == nullptr)
+			  return NULL;
+		  else
+		  {
+			  ListNode *second = head;
+			  while (slow != second)
+			  {
+				  slow=slow->next;
+				  second=second->next;
+
+			  }
+			  return second;
+		  }
+	  }
+  };