Merge pull request #87 from Joshua-Lu/patch-4

更新 0142.环形链表II Java版本

problems/0142.环形链表II.md

@@ -189,33 +189,28 @@ Java：
 ```java
 public class Solution {
     public ListNode detectCycle(ListNode head) {
-        // 1.寻找相遇点
-        ListNode fast = head;
         ListNode slow = head;
-
+        ListNode fast = head;
         while (fast != null && fast.next != null) {
-            fast = fast.next.next;
             slow = slow.next;
-            if (fast != slow) {
+            fast = fast.next.next;
-                continue;
+            if (slow == fast) {// 有环
+                ListNode index1 = fast;
+                ListNode index2 = head;
+                // 两个指针，从头结点和相遇结点，各走一步，直到相遇，相遇点即为环入口
+                while (index1 != index2) {
+                    index1 = index1.next;
+                    index2 = index2.next;
+                }
+                return index1;
             }
-            ListNode meet = fast;
-
-            // 2.寻找入口点
-            slow = head;
-            while (slow != fast) {
-                slow = slow.next;
-                fast = fast.next;
-            }
-
-            return fast;
         }
-
         return null;
     }
 }
 ```
 
+
 Python：
 
 ```python