feat: add solution to lc problem: No.0024 (#4045)

Author: djvelimir

solution/0000-0099/0024.Swap Nodes in Pairs/README.md

@@ -256,6 +256,34 @@ var swapPairs = function (head) {
 };
 ```
 
+#### C#
+
+```cs
+/**
+ * Definition for singly-linked list.
+ * public class ListNode {
+ *     public int val;
+ *     public ListNode next;
+ *     public ListNode(int val=0, ListNode next=null) {
+ *         this.val = val;
+ *         this.next = next;
+ *     }
+ * }
+ */
+public class Solution {
+    public ListNode SwapPairs(ListNode head) {
+        if (head is null || head.next is null) {
+            return head;
+        }
+        ListNode t = SwapPairs(head.next.next);
+        ListNode p = head.next;
+        p.next = head;
+        head.next = t;
+        return p;
+    }
+}
+```
+
 #### Ruby
 
 ```rb
@@ -466,6 +494,38 @@ var swapPairs = function (head) {
 };
 ```
 
+#### C#
+
+```cs
+/**
+ * Definition for singly-linked list.
+ * public class ListNode {
+ *     public int val;
+ *     public ListNode next;
+ *     public ListNode(int val=0, ListNode next=null) {
+ *         this.val = val;
+ *         this.next = next;
+ *     }
+ * }
+ */
+public class Solution {
+    public ListNode SwapPairs(ListNode head) {
+        ListNode dummy = new ListNode(0, head);
+        ListNode pre = dummy;
+        ListNode cur = head;
+        while (cur is not null && cur.next is not null) {
+            ListNode t = cur.next;
+            cur.next = t.next;
+            t.next = cur;
+            pre.next = t;
+            pre = cur;
+            cur = cur.next;
+        }
+        return dummy.next;
+    }
+}
+```
+
 #### PHP
 
 ```php

solution/0000-0099/0024.Swap Nodes in Pairs/README_EN.md

@@ -269,6 +269,34 @@ var swapPairs = function (head) {
 };
 ```
 
+#### C#
+
+```cs
+/**
+ * Definition for singly-linked list.
+ * public class ListNode {
+ *     public int val;
+ *     public ListNode next;
+ *     public ListNode(int val=0, ListNode next=null) {
+ *         this.val = val;
+ *         this.next = next;
+ *     }
+ * }
+ */
+public class Solution {
+    public ListNode SwapPairs(ListNode head) {
+        if (head is null || head.next is null) {
+            return head;
+        }
+        ListNode t = SwapPairs(head.next.next);
+        ListNode p = head.next;
+        p.next = head;
+        head.next = t;
+        return p;
+    }
+}
+```
+
 #### Ruby
 
 ```rb
@@ -479,6 +507,38 @@ var swapPairs = function (head) {
 };
 ```
 
+#### C#
+
+```cs
+/**
+ * Definition for singly-linked list.
+ * public class ListNode {
+ *     public int val;
+ *     public ListNode next;
+ *     public ListNode(int val=0, ListNode next=null) {
+ *         this.val = val;
+ *         this.next = next;
+ *     }
+ * }
+ */
+public class Solution {
+    public ListNode SwapPairs(ListNode head) {
+        ListNode dummy = new ListNode(0, head);
+        ListNode pre = dummy;
+        ListNode cur = head;
+        while (cur is not null && cur.next is not null) {
+            ListNode t = cur.next;
+            cur.next = t.next;
+            t.next = cur;
+            pre.next = t;
+            pre = cur;
+            cur = cur.next;
+        }
+        return dummy.next;
+    }
+}
+```
+
 #### PHP
 
 ```php

solution/0000-0099/0024.Swap Nodes in Pairs/Solution.cs

@@ -0,0 +1,23 @@
+/**
+ * Definition for singly-linked list.
+ * public class ListNode {
+ *     public int val;
+ *     public ListNode next;
+ *     public ListNode(int val=0, ListNode next=null) {
+ *         this.val = val;
+ *         this.next = next;
+ *     }
+ * }
+ */
+public class Solution {
+    public ListNode SwapPairs(ListNode head) {
+        if (head is null || head.next is null) {
+            return head;
+        }
+        ListNode t = SwapPairs(head.next.next);
+        ListNode p = head.next;
+        p.next = head;
+        head.next = t;
+        return p;
+    }
+}

solution/0000-0099/0024.Swap Nodes in Pairs/Solution2.cs

@@ -0,0 +1,27 @@
+/**
+ * Definition for singly-linked list.
+ * public class ListNode {
+ *     public int val;
+ *     public ListNode next;
+ *     public ListNode(int val=0, ListNode next=null) {
+ *         this.val = val;
+ *         this.next = next;
+ *     }
+ * }
+ */
+public class Solution {
+    public ListNode SwapPairs(ListNode head) {
+        ListNode dummy = new ListNode(0, head);
+        ListNode pre = dummy;
+        ListNode cur = head;
+        while (cur is not null && cur.next is not null) {
+            ListNode t = cur.next;
+            cur.next = t.next;
+            t.next = cur;
+            pre.next = t;
+            pre = cur;
+            cur = cur.next;
+        }
+        return dummy.next;
+    }
+}