Update 0024.交换链表节点，添加C#版

problems/0024.两两交换链表中的节点.md

@@ -459,6 +459,40 @@ impl Solution {
 }
 ```
 
+### C#
+```C#
+// 虚拟头结点
+public ListNode SwapPairs(ListNode head)
+{
+    var dummyHead = new ListNode();
+    dummyHead.next = head;
+    ListNode cur = dummyHead;
+    while (cur.next != null && cur.next.next != null)
+    {
+        ListNode tmp1 = cur.next;
+        ListNode tmp2 = cur.next.next.next;
+
+        cur.next = cur.next.next;
+        cur.next.next = tmp1;
+        cur.next.next.next = tmp2;
+
+        cur = cur.next.next;
+    }
+    return dummyHead.next;
+}
+```
+``` C#
+// 递归
+public ListNode SwapPairs(ListNode head)
+{
+    if (head == null || head.next == null) return head;
+    var cur = head.next;
+    head.next = SwapPairs(head.next.next);
+    cur.next = head;
+    return cur;
+}
+```
+
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