1721. Swapping Nodes in a Linked List

algorithms-and-data-structures/src/main/java/com/xtenzq/leetcode/ReverseBetween.java

@@ -0,0 +1,48 @@
+package com.xtenzq.leetcode;
+
+import com.xtenzq.linkedlist.utils.ListNode;
+
+public class ReverseBetween {
+
+    public static ListNode reverseBetween(ListNode head, int left, int right) {
+        if (head.next == null) {
+            return head;
+        }
+
+        ListNode curr = head, beforeList = null;
+        // traverse to the left - 1 node
+        for (int i = 0; i < left - 2; i++) {
+            curr = curr.next;
+        }
+
+        if (left > 1) {
+            beforeList = curr; // the node that will point to the reversed list
+            curr = curr.next; // traversal to the node to start reversing
+        }
+
+        int k = 0;
+        ListNode prev = null, nextNode = null, tail = null;
+        while (k < right - left + 1) {
+            if (k == 0) {
+                tail = curr; // save first node because it will become tail of the reversed list
+            }
+            // reverse node
+            nextNode = curr.next; // save next node
+            curr.next = prev; // reverse direction
+            prev = curr; // prev node is now current
+            curr = nextNode; // traversal
+            k++;
+        }
+
+        // if there are no nodes before the one we have to reverse
+        if (beforeList != null) {
+            beforeList.next = prev;
+        } else {
+            // if we reverse from the start
+            head = prev;
+        }
+
+        tail.next = nextNode;
+        return head;
+    }
+}

algorithms-and-data-structures/src/test/java/com/xtenzq/leetcode/ReverseBetweenTest.java

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+package com.xtenzq.leetcode;
+
+import org.junit.jupiter.api.Test;
+
+import static com.xtenzq.leetcode.ReverseBetween.reverseBetween;
+import static com.xtenzq.linkedlist.utils.ListNode.buildLinkedList;
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+class ReverseBetweenTest {
+
+    @Test
+    void reverseBetween_case1() {
+        assertEquals(buildLinkedList(1, 4, 3, 2, 5), reverseBetween(buildLinkedList(1, 2, 3, 4, 5), 2, 4));
+    }
+
+    @Test
+    void reverseBetween_case2() {
+        assertEquals(buildLinkedList(5), reverseBetween(buildLinkedList(5), 1, 1));
+    }
+
+    @Test
+    void reverseBetween_case3() {
+        assertEquals(buildLinkedList(1, 2), reverseBetween(buildLinkedList(1, 2), 2, 2));
+    }
+
+    @Test
+    void reverseBetween_case4() {
+        assertEquals(buildLinkedList(2, 1), reverseBetween(buildLinkedList(1, 2), 1, 2));
+    }
+}