From 120b28cfd0e8efb966a89a30d16d99dda0e19a9f Mon Sep 17 00:00:00 2001
From: Nikita Rusetskii <rusetscky@outlook.com>
Date: Sun, 21 Jul 2024 20:00:32 +0200
Subject: [PATCH] Add Return the k-th Node from the End Problem (#32)

---
 .../com/xtenzq/linkedlist/FastPointers.java   | 22 +++++++++++++++++++
 .../xtenzq/linkedlist/FastPointersTest.java   | 11 ++++++++++
 2 files changed, 33 insertions(+)

diff --git a/algorithms-and-data-structures/src/main/java/com/xtenzq/linkedlist/FastPointers.java b/algorithms-and-data-structures/src/main/java/com/xtenzq/linkedlist/FastPointers.java
index 0b42a4d..a66d00d 100644
--- a/algorithms-and-data-structures/src/main/java/com/xtenzq/linkedlist/FastPointers.java
+++ b/algorithms-and-data-structures/src/main/java/com/xtenzq/linkedlist/FastPointers.java
@@ -40,4 +40,26 @@ public static boolean hasCycle(ListNode head) {
         }
         return false;
     }
+
+    /**
+     * Return the {@code k}-th node from the end of the linked list
+     *
+     * @param head of linked list
+     * @param k element position from the end
+     * @return node value of {@code k}-th node from the end
+     * @implNote This method runs in {@code O(n)} time complexity and {@code O(1)} space complexity
+     */
+    public static int nodeFromEnd(ListNode head, int k) {
+        ListNode slow = head;
+        ListNode fast = head;
+        for (int i = 0; i < k; i++) {
+            fast = fast.next;
+        }
+
+        while (fast != null) {
+            slow = slow.next;
+            fast = fast.next;
+        }
+        return slow.val;
+    }
 }
diff --git a/algorithms-and-data-structures/src/test/java/com/xtenzq/linkedlist/FastPointersTest.java b/algorithms-and-data-structures/src/test/java/com/xtenzq/linkedlist/FastPointersTest.java
index 2769067..f36c924 100644
--- a/algorithms-and-data-structures/src/test/java/com/xtenzq/linkedlist/FastPointersTest.java
+++ b/algorithms-and-data-structures/src/test/java/com/xtenzq/linkedlist/FastPointersTest.java
@@ -4,6 +4,7 @@
 
 import static com.xtenzq.linkedlist.FastPointers.getMiddle;
 import static com.xtenzq.linkedlist.FastPointers.hasCycle;
+import static com.xtenzq.linkedlist.FastPointers.nodeFromEnd;
 import static com.xtenzq.linkedlist.utils.ListNode.buildLinkedList;
 import static com.xtenzq.linkedlist.utils.ListNode.buildLinkedListWithCycle;
 import static org.junit.jupiter.api.Assertions.assertEquals;
@@ -38,4 +39,14 @@ void hasCycle_case2() {
     void hasCycle_case3() {
         assertFalse(hasCycle(buildLinkedListWithCycle(new int[]{1}, -1)));
     }
+
+    @Test
+    void nodeFromEnd_case1() {
+        assertEquals(4, nodeFromEnd(buildLinkedList(1, 2, 3, 4, 5), 2));
+    }
+
+    @Test
+    void nodeFromEnd_case2() {
+        assertEquals(1, nodeFromEnd(buildLinkedList(1), 1));
+    }
 }
\ No newline at end of file