add solution and test case for 1721

src/main/java/com/fishercoder/solutions/_1721.java

@@ -30,4 +30,42 @@ public ListNode swapNodes(ListNode head, int k) {
             return tmp.next;
         }
     }
+    public static class Solution2 {
+        public ListNode swapNodes(ListNode head, int k) {
+            if(head == null || head.next == null){
+                return head;
+            }
+
+            // find length of list
+            int n = 0;
+            ListNode current = head;
+            while(current != null){
+                current = current.next;
+                n++;
+            }
+
+            int nums[] = new int[n];
+            current = head;
+            int i = 0;
+            while(current != null){
+                nums[i++] = current.val;
+                current = current.next;
+            }
+            int firstIndex = 0;
+            int secondIndex = 0;
+            firstIndex = k;
+            secondIndex = n-k;
+            int temp = nums[firstIndex-1];
+            nums[firstIndex-1] = nums[secondIndex];
+            nums[secondIndex] = temp;
+            ListNode dummy = new ListNode(-1);
+            current = dummy;
+            for(i = 0; i
+                ListNode node = new ListNode(nums[i]);
+                current.next = node;
+                current = current.next;
+            }
+            return dummy.next;
+        }
+    }
 }

src/test/java/com/fishercoder/_1721Test.java

@@ -0,0 +1,38 @@
+package com.fishercoder;
+
+import com.fishercoder.common.classes.ListNode;
+import com.fishercoder.solutions._1721;
+import org.junit.BeforeClass;
+import org.junit.Test;
+
+import static org.junit.Assert.assertEquals;
+
+public class _1721Test {
+    private static _1721.Solution2 solution2;
+    private static ListNode expected;
+    private static ListNode node;
+    private static int k;
+
+    @BeforeClass
+    public static void setup() {
+        solution2 = new _1721.Solution2();
+    }
+
+    @Test
+    public void test1() {
+        node = new ListNode(1);
+        node.next = new ListNode(2);
+        node.next.next = new ListNode(3);
+        node.next.next.next = new ListNode(4);
+        node.next.next.next.next = new ListNode(5);
+
+        expected = new ListNode(1);
+        expected.next = new ListNode(4);
+        expected.next.next = new ListNode(3);
+        expected.next.next.next = new ListNode(2);
+        expected.next.next.next.next = new ListNode(5);
+
+        k = 2;
+        assertEquals(expected, solution2.swapNodes(node, k));
+    }
+}