Add Return the k-th Node from the End Problem (#32)

xtenzQ · Jul 21, 2024 · 120b28c · 120b28c
1 parent 00632d4
commit 120b28c
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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
@@ -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
@@ -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));
+    }
 }