Update 0019.删除链表的倒数第N个节点.md

增加C语言版本，已通过Leetcode验证

problems/0019.删除链表的倒数第N个节点.md

@@ -364,6 +364,40 @@ impl Solution {
         dummy_head.next
     }
 }
+```
+C语言
+```c
+/**c语言单链表的定义
+ * Definition for singly-linked list.
+ * struct ListNode {
+ *     int val;
+ *     struct ListNode *next;
+ * };
+ */
+struct ListNode* removeNthFromEnd(struct ListNode* head, int n) {
+    //定义虚拟头节点dummy 并初始化使其指向head
+    struct ListNode* dummy = malloc(sizeof(struct ListNode));
+    dummy->val = 0;
+    dummy->next = head;
+    //定义 fast slow 双指针
+    struct ListNode* fast = head;
+    struct ListNode* slow = dummy;
+
+    for (int i = 0; i < n; ++i) {
+        fast = fast->next;
+    }
+    while (fast) {
+        fast = fast->next;
+        slow = slow->next;
+    }
+    slow->next = slow->next->next;//删除倒数第n个节点
+    head = dummy->next;
+    free(dummy);//删除虚拟节点dummy
+    return head;
+}
+
+
+
 ```
 
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