添加142.环形链表IIjavascript版本

problems/0142.环形链表II.md

@@ -258,6 +258,50 @@ Go：
 }
 ```
 
+javaScript
+
+```js
+// 两种循环实现方式
+/**
+ * @param {ListNode} head
+ * @return {ListNode}
+ */
+// 先判断是否是环形链表
+var detectCycle = function(head) {
+    if(!head || !head.next) return null;
+    let slow =head.next, fast = head.next.next;
+    while(fast && fast.next && fast!== slow) {
+        slow = slow.next;
+        fast = fast.next.next; 
+    }
+    if(!fast || !fast.next ) return null;
+    slow = head;
+    while (fast !== slow) {
+        slow = slow.next;
+        fast = fast.next;
+    }
+    return slow;
+};
+
+var detectCycle = function(head) {
+    if(!head || !head.next) return null;
+    let slow =head.next, fast = head.next.next;
+    while(fast && fast.next) {
+        slow = slow.next;
+        fast = fast.next.next;
+        if(fast == slow) {
+            slow = head;
+            while (fast !== slow) {
+                slow = slow.next;
+                fast = fast.next;
+            }
+            return slow;
+        }
+    }
+    return null;
+};
+```
+
 
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