JavaScript Program to Check If a Singly Linked List is Palindrome

A singly linked list is a linear data structure that is stored in a non-contiguous way in the memory and each block is connected by holding the address of the next block also known as a node. A palindrome can be explained as a set of characters, digits, etc, and it reads the same from both the front and backside. We will be given a singly linked list and have to find whether the values stored at the nodes are equal from both the front and backside.

Input

1 -> 2 -> 3 -> 3 -> 2 -> 1 -> null

Output

Yes, the given linked list is a palindrome. 

Explanation

We can see the first and the last node have the same value, the second and second last, and so on, each node at the same distance from the front and backside have the same value.

Input

1 -> 2 -> 3 -> 4 -> 2 -> 1 -> null

Output

No, the given linked list is not a palindrome. 

Explanation

Here, the first and second nodes are equal to the last and second last node respectively but after that nodes don’t have the same value.

Using Stack

In this approach, we will first create a linked list by using the class and then will define some basic functions to add the data to the linked list and to print the data present in the linked list.

Example

// class to create the structure of the nodes 
class Node{
   constructor(data){
      this.value = data;
      this.next = null;
   }
}

// function to print the linked list
function print(head){
   var temp = head;
   var ans = "";    
   while(temp.next != null){
      ans += temp.value;
      ans += " -> "
      temp = temp.next
   }
   ans += temp.value
   ans += " -> null"
   console.log(ans)
}

// function to add data in linked list 
function add(data, head, tail){
   return tail.next = new Node(data);
}

// function to find the string is palindrome or not
function check(head){
   var temp = head;
   var stack = []; // defining the stack    
   while(temp != null){
      stack.push(temp.value);
      temp = temp.next;
   }    
   temp = head;
   while(temp != null){
      if(temp.value != stack.pop()){
         return false;
      }
      temp = temp.next;
   }
   return true;
}

// defining linked list
var head  = new Node(1)
var tail  = head
tail = add(2,head, tail)
tail = add(3,head, tail)
tail = add(3,head, tail)
tail = add(2,head, tail)
tail = add(1,head, tail)
console.log("The given linked list is: ")
print(head)

// calling function to check if the current linked list is a palindrome or not 
if(check(head)){
   console.log("Yes, the given linked list is a palindrome");
}
else{
   console.log("No, the given linked list is not a palindrome");
}

Time and Space Complexity

The time complexity of the above code is O(N) where N is the length of the linked list.

The space complexity of the above code is O(N), as we are using a stack data structure to store the elements in it.

Using Recursion

In this method, we will first find the length of the given linked list and then we will traverse to the mid of the linked list using the recursion. If the length of the given linked list odd, we will return next of the mid node else mid node and for each call we will get the respective node from the back from recursive call.

Example

// class to create the structure of the nodes 
class Node{
   constructor(data){
      this.value = data;
      this.next = null;
   }
}

// function to print the linked list
function print(head){
   var temp = head;
   var ans = "";    
   while(temp.next != null){
      ans += temp.value;
      ans += " -> "
      temp = temp.next
   }
   ans += temp.value
   ans += " -> null"
   console.log(ans)
}

// function to add data in linked list 
function add(data, head, tail){
   return tail.next = new Node(data);
}

// recursive function 
function recursion(head, number, odd){
   if(number == 0){
      if(odd){
         return head.next;
      }
      else{
         return head;
      }
   }
   var temp = recursion(head.next, number-1, odd);
   
   // if the current value is not equal then don't move to the next node
   
   // by this we will not reach null in the end 
   
   // indicated the not palindrome 
   
   if(temp.value != head.value){
      return temp;
   }
   else{
      return temp.next;
   }
}

// function to check if the given linked list is palindrome or not 
function check(head){
   var temp = head;
   var len = 0;

   // finding the length of the given linked list 
   while(temp != null){
      len++;
      temp = temp.next;
   }

   // calling the recursion function 
   if(recursion(head, Math.floor(len/2), len & 1) == null){
      return true;
   }
   else{
      return false;
   }
}

// defining linked list
var head  = new Node(1)
var tail  = head
tail = add(2,head, tail)
tail = add(3,head, tail)
tail = add(4,head, tail)
tail = add(3,head, tail)
tail = add(2,head, tail)
tail = add(1,head, tail)
console.log("The given linked list is: ")
print(head)

// calling function to check if the current linked list is a palindrome or not 
if(check(head)){
   console.log("Yes, the given linked list is a palindrome");
}
else{
   console.log("No, the given linked list is not a palindrome");
}

Conclusion

In this tutorial, we have implemented a JavaScript program to check if the given linked list nodes contain the values in the palindrome or not. We have implemented two codes using the stack and the recursion which works with the time complexity of the O(N) and space complexity is O(N) for stack and O(1) for the recursive method (only if the recursive call data is not considered).