0142.环形链表II：添加C语言实现

problems/0142.环形链表II.md

@@ -328,5 +328,26 @@ extension ListNode: Equatable {
 }
 ```
 
+C：
+
+```c
+ListNode *detectCycle(ListNode *head) {
+    ListNode *fast = head, *slow = head;
+    while (fast && fast->next) {
+        // 这里判断两个指针是否相等，所以移位操作放在前面
+        slow = slow->next;
+        fast = fast->next->next;
+        if (slow == fast) { // 相交，开始找环形入口：分别从头部和从交点出发，找到相遇的点就是环形入口
+            ListNode *f = fast, *h = head;
+            while (f != h) f = f->next, h = h->next;
+            return h;
+        }
+    }
+    return NULL;
+}
+```
+
+
+
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