Add palindrome singly linkedlist optimised approach (#5617)

Author: Nandini-Pandey

src/main/java/com/thealgorithms/misc/PalindromeSinglyLinkedList.java

@@ -30,4 +30,47 @@ public static boolean isPalindrome(final SinglyLinkedList linkedList) {
 
         return true;
     }
+
+    // Optimised approach with O(n) time complexity and O(1) space complexity
+
+    public static boolean isPalindromeOptimised(Node head) {
+        if (head == null || head.next == null) {
+            return true;
+        }
+        Node slow = head;
+        Node fast = head;
+        while (fast != null && fast.next != null) {
+            slow = slow.next;
+            fast = fast.next.next;
+        }
+        Node midNode = slow;
+
+        Node prevNode = null;
+        Node currNode = midNode;
+        Node nextNode;
+        while (currNode != null) {
+            nextNode = currNode.next;
+            currNode.next = prevNode;
+            prevNode = currNode;
+            currNode = nextNode;
+        }
+        Node left = head;
+        Node right = prevNode;
+        while (left != null && right != null) {
+            if (left.val != right.val) {
+                return false;
+            }
+            right = right.next;
+            left = left.next;
+        }
+        return true;
+    }
+    static class Node {
+        int val;
+        Node next;
+        Node(int val) {
+            this.val = val;
+            this.next = null;
+        }
+    }
 }

src/test/java/com/thealgorithms/misc/PalindromeSinglyLinkedListTest.java

@@ -7,6 +7,8 @@
 import org.junit.jupiter.api.Test;
 
 public class PalindromeSinglyLinkedListTest {
+
+    // Stack-based tests
     @Test
     public void testWithEmptyList() {
         assertTrue(PalindromeSinglyLinkedList.isPalindrome(new SinglyLinkedList()));
@@ -67,4 +69,74 @@ public void testWithListWithEvenLengthNegative() {
         exampleList.insert(20);
         assertFalse(PalindromeSinglyLinkedList.isPalindrome(exampleList));
     }
+
+    // Optimized approach tests
+    @Test
+    public void testOptimisedWithEmptyList() {
+        assertTrue(PalindromeSinglyLinkedList.isPalindromeOptimised(null));
+    }
+
+    @Test
+    public void testOptimisedWithSingleElement() {
+        PalindromeSinglyLinkedList.Node node = new PalindromeSinglyLinkedList.Node(100);
+        assertTrue(PalindromeSinglyLinkedList.isPalindromeOptimised(node));
+    }
+
+    @Test
+    public void testOptimisedWithOddLengthPositive() {
+        PalindromeSinglyLinkedList.Node node1 = new PalindromeSinglyLinkedList.Node(1);
+        PalindromeSinglyLinkedList.Node node2 = new PalindromeSinglyLinkedList.Node(2);
+        PalindromeSinglyLinkedList.Node node3 = new PalindromeSinglyLinkedList.Node(1);
+        node1.next = node2;
+        node2.next = node3;
+        assertTrue(PalindromeSinglyLinkedList.isPalindromeOptimised(node1));
+    }
+
+    @Test
+    public void testOptimisedWithOddLengthPositive2() {
+        PalindromeSinglyLinkedList.Node node1 = new PalindromeSinglyLinkedList.Node(3);
+        PalindromeSinglyLinkedList.Node node2 = new PalindromeSinglyLinkedList.Node(2);
+        PalindromeSinglyLinkedList.Node node3 = new PalindromeSinglyLinkedList.Node(1);
+        PalindromeSinglyLinkedList.Node node4 = new PalindromeSinglyLinkedList.Node(2);
+        PalindromeSinglyLinkedList.Node node5 = new PalindromeSinglyLinkedList.Node(3);
+        node1.next = node2;
+        node2.next = node3;
+        node3.next = node4;
+        node4.next = node5;
+        assertTrue(PalindromeSinglyLinkedList.isPalindromeOptimised(node1));
+    }
+
+    @Test
+    public void testOptimisedWithEvenLengthPositive() {
+        PalindromeSinglyLinkedList.Node node1 = new PalindromeSinglyLinkedList.Node(10);
+        PalindromeSinglyLinkedList.Node node2 = new PalindromeSinglyLinkedList.Node(20);
+        PalindromeSinglyLinkedList.Node node3 = new PalindromeSinglyLinkedList.Node(20);
+        PalindromeSinglyLinkedList.Node node4 = new PalindromeSinglyLinkedList.Node(10);
+        node1.next = node2;
+        node2.next = node3;
+        node3.next = node4;
+        assertTrue(PalindromeSinglyLinkedList.isPalindromeOptimised(node1));
+    }
+
+    @Test
+    public void testOptimisedWithOddLengthNegative() {
+        PalindromeSinglyLinkedList.Node node1 = new PalindromeSinglyLinkedList.Node(1);
+        PalindromeSinglyLinkedList.Node node2 = new PalindromeSinglyLinkedList.Node(2);
+        PalindromeSinglyLinkedList.Node node3 = new PalindromeSinglyLinkedList.Node(2);
+        node1.next = node2;
+        node2.next = node3;
+        assertFalse(PalindromeSinglyLinkedList.isPalindromeOptimised(node1));
+    }
+
+    @Test
+    public void testOptimisedWithEvenLengthNegative() {
+        PalindromeSinglyLinkedList.Node node1 = new PalindromeSinglyLinkedList.Node(10);
+        PalindromeSinglyLinkedList.Node node2 = new PalindromeSinglyLinkedList.Node(20);
+        PalindromeSinglyLinkedList.Node node3 = new PalindromeSinglyLinkedList.Node(20);
+        PalindromeSinglyLinkedList.Node node4 = new PalindromeSinglyLinkedList.Node(20);
+        node1.next = node2;
+        node2.next = node3;
+        node3.next = node4;
+        assertFalse(PalindromeSinglyLinkedList.isPalindromeOptimised(node1));
+    }
 }