Javascript Program For Checking Linked List With A Loop Is Palindrome Or Not Last Updated : 03 Sep, 2024 Comments Improve Suggest changes Like Article Like Report Given a linked list with a loop, the task is to find whether it is palindrome or not. You are not allowed to remove the loop. Examples: Input: 1 -> 2 -> 3 -> 2 /| |/ ------- 1 Output: PalindromeLinked list is 1 2 3 2 1 which is a palindrome.Input: 1 -> 2 -> 3 -> 4 /| |/ ------- 1 Output: Not PalindromeLinked list is 1 2 3 4 1 which is a not palindrome. Algorithm:Detect the loop using the Floyd Cycle Detection Algorithm.Then find the starting node of the loop as discussed in this.Check the linked list is palindrome or not as discussed in this.Below is the implementation. JavaScript // Javascript program to check if a // linked list with loop is palindrome // or not. // Link list node class Node { constructor() { this.data = 0; this.next = null; } } /* Function to find loop starting node. loop_node --> Pointer to one of the loop nodes head --> Pointer to the start node of the linked list */ function getLoopstart(loop_node, head) { let ptr1 = loop_node; let ptr2 = loop_node; // Count the number of nodes in // loop let k = 1, i; while (ptr1.next != ptr2) { ptr1 = ptr1.next; k++; } // Fix one pointer to head ptr1 = head; // And the other pointer to k // nodes after head ptr2 = head; for (i = 0; i < k; i++) ptr2 = ptr2.next; /* Move both pointers at the same pace, they will meet at loop starting node */ while (ptr2 != ptr1) { ptr1 = ptr1.next; ptr2 = ptr2.next; } return ptr1; } /* This function detects and find loop starting node in the list */ function detectAndgetLoopstarting(head) { let slow_p = head, fast_p = head, loop_start = null; // Start traversing list and detect loop while (slow_p != null && fast_p != null && fast_p.next != null) { slow_p = slow_p.next; fast_p = fast_p.next.next; /* If slow_p and fast_p meet then find the loop starting node */ if (slow_p == fast_p) { loop_start = getLoopstart(slow_p, head); break; } } // Return starting node of loop return loop_start; } // Utility function to check if a linked // list with loop is palindrome with given // starting point. function isPalindromeUtil(head, loop_start) { let ptr = head; let s = []; // Traverse linked list until last node // is equal to loop_start and store the // elements till start in a stack let count = 0; while (ptr != loop_start || count != 1) { s.push(ptr.data); if (ptr == loop_start) count = 1; ptr = ptr.next; } ptr = head; count = 0; // Traverse linked list until last // node is equal to loop_start second // time while (ptr != loop_start || count != 1) { // Compare data of node with // the top of stack // If equal then continue let stk = s.pop(); if (ptr.data == stk); // Else return false else { s.push(stk); return false; } if (ptr == loop_start) count = 1; ptr = ptr.next; } // Return true if linked list is // palindrome return true; } // Function to find if linked list // is palindrome or not function isPalindrome(head) { // Find the loop starting node let loop_start = detectAndgetLoopstarting(head); // Check if linked list is palindrome return isPalindromeUtil(head, loop_start); } function newNode(key) { let temp = new Node(); temp.data = key; temp.next = null; return temp; } // Driver code let head = newNode(50); head.next = newNode(20); head.next.next = newNode(15); head.next.next.next = newNode(20); head.next.next.next.next = newNode(50); // Create a loop for testing head.next.next.next.next.next = head.next.next; if (isPalindrome(head) == true) console.log("Palindrome"); else console.log("Not Palindrome"); // This code is contributed by aashish1995 OutputPalindrome Complexity Analysis:Time Complexity: O(n) where n is no of nodes in the linked listAuxiliary Space: O(n) where n is size of ListPlease refer complete article on Check linked list with a loop is palindrome or not for more details! 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