Add a problem 'Reverse Linked List'

Author: juchanei

jvm/src/main/java/io/github/juchanei/leetcodeJava/ReverseLinkedList.kt

@@ -0,0 +1,76 @@
+package io.github.juchanei.leetcodeJava
+
+import org.assertj.core.api.BDDAssertions.then
+import org.junit.jupiter.api.Test
+
+// https://leetcode.com/explore/interview/card/top-interview-questions-easy/93/linked-list/560/
+class ReverseLinkedList {
+    data class ListNode(val `val`: Int, val next: ListNode?)
+
+    fun reverseList(head: ListNode?): ListNode? =
+        reverseList(head = head, acc = null)
+
+    private tailrec fun reverseList(head: ListNode?, acc: ListNode?): ListNode? =
+        if (head == null) {
+            acc
+        } else {
+            reverseList(
+                head = head.next,
+                acc = ListNode(head.`val`, acc)
+            )
+        }
+
+    @Test
+    fun example1() {
+        val l = listOf(1, 2)
+
+        val actual = reverseList(l.toListNode())
+
+        then(actual.toList()).isEqualTo(l.reversed())
+    }
+
+    @Test
+    fun example2() {
+        val l = emptyList<Int>()
+
+        val actual = reverseList(l.toListNode())
+
+        then(actual.toList()).isEqualTo(l.reversed())
+    }
+
+    @Test
+    fun example3() {
+        val l = listOf(1)
+
+        val actual = reverseList(l.toListNode())
+
+        then(actual.toList()).isEqualTo(l.reversed())
+    }
+
+    @Test
+    fun example4() {
+        val l = listOf(1, 2, 3, 4, 5, 6, 7, 8, 9, 0)
+
+        val actual = reverseList(l.toListNode())
+
+        then(actual.toList()).isEqualTo(l.reversed())
+    }
+
+    private fun List<Int>.toListNode(): ListNode? {
+        if (isEmpty()) return null
+        return ListNode(first(), drop(1).toListNode())
+    }
+
+    private fun ListNode?.toList(): List<Int> {
+        val result = mutableListOf<Int>()
+
+        var node: ListNode? = this
+        while (true) {
+            if (node == null) break
+            result.add(node.`val`)
+            node = node.next
+        }
+
+        return result
+    }
+}