Add palindrome singly linkedlist optimised approach (#5617)

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

@@ -30,4 +30,47 @@ public final class PalindromeSinglyLinkedList {
 
         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;
+        }
+    }
 }