cs: add «143. Reorder List»

Signed-off-by: Matej Focko <me@mfocko.xyz>

cs/reorder-list.cs

@@ -0,0 +1,38 @@
+using System.Collections.Generic;
+
+/**
+ * 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 {
+    private List<ListNode> ListAsList(ListNode head) {
+        var result = new List<ListNode>();
+
+        for (var node = head; node != null; node = node.next) {
+            result.Add(node);
+        }
+
+        return result;
+    }
+
+    public void ReorderList(ListNode head) {
+        var nodes = ListAsList(head);
+
+        ListNode node = head;
+        for (int i = 1, j = nodes.Count - 1; i <= j; ++i, --j) {
+            node.next = nodes[j];
+            node = node.next;
+
+            node.next = nodes[i];
+            node = node.next;
+        }
+        node.next = null;
+    }
+}