Javascript Program For Writing A Function To Get Nth Node In A Linked List

Write a GetNth() function that takes a linked list and an integer index and returns the data value stored in the node at that index position. 

Example:

Input:  1 -> 10 -> 30 -> 14,  index = 2
Output: 30  
Explanation: The node at index 2 is 30

Method 1 – Using Loop

Algorithm:

1. Initialize count = 0
2. Loop through the link list
     a. If count is equal to the passed index then return current node
     b. Increment count
     c. change current to point to next of the current.

Implementation:

Javascript

// Javascript program to find nth
// node of a linked list

class Node {
    constructor(d) {
        this.data = d;
        this.next = null;
    }
}

// Head of list
let head;

/* Takes index as argument and
   return data at index */
function GetNth(index) {
    let current = head;

    // Index of Node we are currently
    // looking at
    let count = 0;

    while (current != null) {
        if (count == index)
            return current.data;
        count++;
        current = current.next;
    }

    /* if we get to this line, the caller
       was asking for a non-existent element
       so we assert fail */
    assert(false);
    return 0;
}

/* Given a reference to the head of a
   list and an int, inserts a new Node
   on the front of the list */
function push(new_data) {

    // 1. Create new Node and put data
    let new_Node = new Node(new_data);

    // 2. Make next of new Node as head
    new_Node.next = head;

    // 3. Move the head to point to new Node
    head = new_Node;
}

// Driver code

// Start with empty list
// Use push() method to construct list
// 1 -> 12 -> 1 -> 4 -> 1
push(1);
push(4);
push(1);
push(12);
push(1);

// Check the count function
console.log("Element at index 3 is " +
    GetNth(3));

Output

Element at index 3 is 4

Time Complexity: O(n)

Space Complexity: O(1) since using constant space to create nodes and variables.

Method 2 – Using Recursion

Algorithm: 

getnth(node,n)
1. Initialize count = 0
2. if count==n
     return node->data
3. else
    return getnth(node->next, n-1)

Implementation:

Javascript

// JavaScript program to find n'th node
// of a linked list using recursion

// Linked List node
class Node {
    constructor(val) {
        this.data = val;
        this.next = null;
    }
}

/* Given a reference (pointer to pointer) to the
   head of a list and an int, push a new node on
   the front of the list. */
function push(head, new_data) {

    // Allocate node
    let new_node = new Node(new_data);

    // Put in the data
    new_node.data = new_data;

    new_node.next = head;
    head = new_node;
    return head;
}

/* Takes head pointer of the linked list and
   index as arguments and return data at index */
function GetNth(head, n) {
    let count = 0;

    // Edge case - if head is null
    if (head == null)
        return -1;

    // if count equal too n return node.data
    if (count == n)
        return head.data;

    // Recursively decrease n and increase
    // head to next pointer
    return GetNth(head.next, n - 1);
}

// Driver code

// Start with the empty list
let head = null;

// Use push() to construct list
// 1 -> 12 -> 1 -> 4 -> 1
head = push(head, 1);
head = push(head, 4);
head = push(head, 1);
head = push(head, 12);
head = push(head, 1);

// Check the count function
console.log("Element at index 3 is ",
    GetNth(head, 3));

Output

Element at index 3 is  4

Time Complexity: O(n) 

Space Complexity: O(n) for call stack because using recursion

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