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Add palindrome singly linkedlist optimised approach (#5617)
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@ -30,4 +30,47 @@ public final class PalindromeSinglyLinkedList {
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return true;
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return true;
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}
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}
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// Optimised approach with O(n) time complexity and O(1) space complexity
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public static boolean isPalindromeOptimised(Node head) {
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if (head == null || head.next == null) {
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return true;
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}
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Node slow = head;
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Node fast = head;
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while (fast != null && fast.next != null) {
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slow = slow.next;
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fast = fast.next.next;
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}
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Node midNode = slow;
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Node prevNode = null;
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Node currNode = midNode;
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Node nextNode;
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while (currNode != null) {
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nextNode = currNode.next;
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currNode.next = prevNode;
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prevNode = currNode;
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currNode = nextNode;
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}
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Node left = head;
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Node right = prevNode;
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while (left != null && right != null) {
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if (left.val != right.val) {
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return false;
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}
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right = right.next;
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left = left.next;
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}
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return true;
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}
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static class Node {
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int val;
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Node next;
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Node(int val) {
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this.val = val;
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this.next = null;
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}
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}
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}
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}
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@ -7,6 +7,8 @@ import com.thealgorithms.datastructures.lists.SinglyLinkedList;
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import org.junit.jupiter.api.Test;
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import org.junit.jupiter.api.Test;
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public class PalindromeSinglyLinkedListTest {
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public class PalindromeSinglyLinkedListTest {
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// Stack-based tests
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@Test
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@Test
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public void testWithEmptyList() {
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public void testWithEmptyList() {
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assertTrue(PalindromeSinglyLinkedList.isPalindrome(new SinglyLinkedList()));
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assertTrue(PalindromeSinglyLinkedList.isPalindrome(new SinglyLinkedList()));
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@ -67,4 +69,74 @@ public class PalindromeSinglyLinkedListTest {
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exampleList.insert(20);
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exampleList.insert(20);
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assertFalse(PalindromeSinglyLinkedList.isPalindrome(exampleList));
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assertFalse(PalindromeSinglyLinkedList.isPalindrome(exampleList));
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}
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}
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// Optimized approach tests
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@Test
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public void testOptimisedWithEmptyList() {
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assertTrue(PalindromeSinglyLinkedList.isPalindromeOptimised(null));
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}
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@Test
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public void testOptimisedWithSingleElement() {
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PalindromeSinglyLinkedList.Node node = new PalindromeSinglyLinkedList.Node(100);
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assertTrue(PalindromeSinglyLinkedList.isPalindromeOptimised(node));
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}
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@Test
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public void testOptimisedWithOddLengthPositive() {
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PalindromeSinglyLinkedList.Node node1 = new PalindromeSinglyLinkedList.Node(1);
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PalindromeSinglyLinkedList.Node node2 = new PalindromeSinglyLinkedList.Node(2);
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PalindromeSinglyLinkedList.Node node3 = new PalindromeSinglyLinkedList.Node(1);
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node1.next = node2;
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node2.next = node3;
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assertTrue(PalindromeSinglyLinkedList.isPalindromeOptimised(node1));
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}
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@Test
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public void testOptimisedWithOddLengthPositive2() {
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PalindromeSinglyLinkedList.Node node1 = new PalindromeSinglyLinkedList.Node(3);
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PalindromeSinglyLinkedList.Node node2 = new PalindromeSinglyLinkedList.Node(2);
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PalindromeSinglyLinkedList.Node node3 = new PalindromeSinglyLinkedList.Node(1);
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PalindromeSinglyLinkedList.Node node4 = new PalindromeSinglyLinkedList.Node(2);
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PalindromeSinglyLinkedList.Node node5 = new PalindromeSinglyLinkedList.Node(3);
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node1.next = node2;
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node2.next = node3;
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node3.next = node4;
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node4.next = node5;
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assertTrue(PalindromeSinglyLinkedList.isPalindromeOptimised(node1));
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}
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@Test
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public void testOptimisedWithEvenLengthPositive() {
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PalindromeSinglyLinkedList.Node node1 = new PalindromeSinglyLinkedList.Node(10);
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PalindromeSinglyLinkedList.Node node2 = new PalindromeSinglyLinkedList.Node(20);
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PalindromeSinglyLinkedList.Node node3 = new PalindromeSinglyLinkedList.Node(20);
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PalindromeSinglyLinkedList.Node node4 = new PalindromeSinglyLinkedList.Node(10);
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node1.next = node2;
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node2.next = node3;
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node3.next = node4;
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assertTrue(PalindromeSinglyLinkedList.isPalindromeOptimised(node1));
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}
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@Test
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public void testOptimisedWithOddLengthNegative() {
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PalindromeSinglyLinkedList.Node node1 = new PalindromeSinglyLinkedList.Node(1);
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PalindromeSinglyLinkedList.Node node2 = new PalindromeSinglyLinkedList.Node(2);
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PalindromeSinglyLinkedList.Node node3 = new PalindromeSinglyLinkedList.Node(2);
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node1.next = node2;
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node2.next = node3;
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assertFalse(PalindromeSinglyLinkedList.isPalindromeOptimised(node1));
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}
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@Test
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public void testOptimisedWithEvenLengthNegative() {
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PalindromeSinglyLinkedList.Node node1 = new PalindromeSinglyLinkedList.Node(10);
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PalindromeSinglyLinkedList.Node node2 = new PalindromeSinglyLinkedList.Node(20);
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PalindromeSinglyLinkedList.Node node3 = new PalindromeSinglyLinkedList.Node(20);
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PalindromeSinglyLinkedList.Node node4 = new PalindromeSinglyLinkedList.Node(20);
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node1.next = node2;
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node2.next = node3;
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node3.next = node4;
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assertFalse(PalindromeSinglyLinkedList.isPalindromeOptimised(node1));
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}
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}
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}
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