doubleIt.js

/**
 * Definition for singly-linked list.
 * function ListNode(val, next) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.next = (next===undefined ? null : next)
 * }
 */
/**
 * @param {ListNode} head
 * @return {ListNode}
 */
var doubleIt = function(head) {
    // Initialize a dummy head for the new linked list
    var dummyHead = new ListNode(0);
    var current = dummyHead;
    
    // Initialize a variable to store the carry
    var carry = 0;
    
    // Traverse the linked list from the least significant digit
    var node = reverseList(head);
    
    while (node != null || carry > 0) {
        // Double the current digit and add the carry
        var sum = (node != null ? node.val : 0) * 2 + carry;
        
        // Update the carry
        carry = Math.floor(sum / 10);
        
        // Create a new node for the current digit
        current.next = new ListNode(sum % 10);
        
        // Move to the next digit
        current = current.next;
        if (node != null) node = node.next;
    }
    
    // Reverse the linked list to get the correct order
    return reverseList(dummyHead.next);
};

// Helper function to reverse a linked list
function reverseList(head) {
    var prev = null;
    var current = head;
    while (current != null) {
        var nextTemp = current.next;
        current.next = prev;
        prev = current;
        current = nextTemp;
    }
    return prev;
}