add a solution for 24

fishercoder1534 · fishercoder1534 · commit 8b9c165cc421 · 2021-10-24T14:10:44.000-07:00
diff --git a/src/main/java/com/fishercoder/solutions/_24.java b/src/main/java/com/fishercoder/solutions/_24.java
@@ -4,6 +4,9 @@
 
 public class _24 {
     public static class Solution1 {
+        /**
+         * Recursive solution.
+         */
         public ListNode swapPairs(ListNode head) {
             if (head == null || head.next == null) {
                 return head;
@@ -16,4 +19,32 @@ public ListNode swapPairs(ListNode head) {
         }
     }
 
+    public static class Solution2 {
+        /**
+         * Iterative approach:
+         * My completely original on 10/24/2021.
+         */
+        public ListNode swapPairs(ListNode head) {
+            ListNode pre = new ListNode(-1);
+            pre.next = head;
+            ListNode tmp = pre;
+            while (head != null) {
+                ListNode third;
+                ListNode first = head;
+                ListNode second = head.next;
+                if (second == null) {
+                    break;
+                } else {
+                    third = head.next.next;
+                    second.next = first;
+                    first.next = third;
+                    tmp.next = second;
+                    tmp = tmp.next.next;
+                }
+                head = third;
+            }
+            return pre.next;
+        }
+    }
+
 }
diff --git a/src/test/java/com/fishercoder/_24Test.java b/src/test/java/com/fishercoder/_24Test.java
@@ -1,27 +1,39 @@
 package com.fishercoder;
 
 import com.fishercoder.common.classes.ListNode;
-import com.fishercoder.common.utils.CommonUtils;
 import com.fishercoder.common.utils.LinkedListUtils;
 import com.fishercoder.solutions._24;
 import org.junit.BeforeClass;
 import org.junit.Test;
 
 import java.util.Arrays;
 
+import static org.junit.Assert.assertEquals;
+
 public class _24Test {
     private static _24.Solution1 solution1;
+    private static _24.Solution2 solution2;
     private static ListNode head;
+    private static ListNode expected;
 
     @BeforeClass
     public static void setup() {
         solution1 = new _24.Solution1();
+        solution2 = new _24.Solution2();
     }
 
     @Test
     public void test1() {
         head = LinkedListUtils.createSinglyLinkedList(Arrays.asList(1, 2, 3, 4));
-        CommonUtils.printList(solution1.swapPairs(head));
+        expected = LinkedListUtils.createSinglyLinkedList(Arrays.asList(2, 1, 4, 3));
+        assertEquals(expected, solution1.swapPairs(head));
+    }
+
+    @Test
+    public void test2() {
+        head = LinkedListUtils.createSinglyLinkedList(Arrays.asList(1, 2, 3, 4));
+        expected = LinkedListUtils.createSinglyLinkedList(Arrays.asList(2, 1, 4, 3));
+        assertEquals(expected, solution2.swapPairs(head));
     }
 
 }
