cs: add «3217. Delete Nodes From Linked List Present in Array»

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

cs/ListNode.cs

@@ -0,0 +1,9 @@
+public class ListNode {
+    public int val;
+    public ListNode? next;
+
+    public ListNode(int val = 0, ListNode? next = null) {
+        this.val = val;
+        this.next = next;
+    }
+}

cs/delete-nodes-from-linked-list-present-in-array.cs

@@ -0,0 +1,21 @@
+using System.Collections.Generic;
+
+public class Solution {
+    public ListNode ModifiedList(int[] nums, ListNode head) {
+        var uniqueNums = new HashSet<int>(nums);
+
+        var node = head;
+        while (node.next != null) {
+            if (uniqueNums.Contains(node.next.val)) {
+                node.next = node.next.next;
+            } else {
+                node = node.next;
+            }
+        }
+
+        if (uniqueNums.Contains(head.val)) {
+            return head.next;
+        }
+        return head;
+    }
+}