Leetcode Add Two Numbers Q: how to create a linkedlist from a number?

Question

In this question: https://leetcode.com/problems/add-two-numbers/description/, I've figured out how to get sum of the digits (i.e. 807), but now I need to convert that number into a linked list (e.g. 7 -> 0 -> 8).

How do I create a linkedlist from a number?

Class function for creating a list node:

function ListNode(val) {
   this.val = val;
   this.next = null;
}

Rest of my code:

var addTwoNumbers = function(l1, l2) {

    function findVal(linkedList) {
          return linkedList.next == null ? linkedList.val.toString() : linkedList.val.toString() + findVal(linkedList.next);

    }

    var l1num = parseInt(findVal(l1).split('').reverse().join(''));
    var l2num = parseInt(findVal(l2).split('').reverse().join(''));
    var sum = l1num + l2num;

// Create linked list from sum

user184994 · Accepted Answer · 2017-12-07 06:03:19Z

3

If you turn your number into an array, you can then use the array.prototype.reduce function.

let number = 807;

function ListNode(val) {
  this.val = val;
  this.next = null;
}


// Convert the number into a string and split that into an array of digits
let arr = Array.from(number.toString());

// Iterate through the elements, and create the nodes
let head = arr.reduce((nextNode, curr) => {
  let node = new ListNode(curr);
  node.next = nextNode;
  return node;
}, null);


// Print out the values
let node = head;
while(node) {
  console.log(node.val);
  node = node.next
}

edited Dec 7, 2017 at 6:03

answered Dec 6, 2017 at 18:56

user184994

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7 Comments

Bergi Over a year ago

You really shouldn't need that slice(1). Just pass null as the initial value and use reduceRight.

Thomas Over a year ago

@user184994, almost. The LL are supposed to be in the opposite order. Starting with the ones, then the tens, hundreds, ... so just change the reduceRight back to a reduce and we should be fine ;)

Bergi Over a year ago

@Thomas Oh, I didn't notice the OP needs the list backwards.

user184994 Over a year ago

@Bergi This no longer works following your latest edit- head is undefined. You'll need to call new ListNode as opposed to ListNode

Bergi Over a year ago

@user184994 Ouch, I didn't mean to remove the new. Thanks for testing :-)

|

Slai · Accepted Answer · 2017-12-06 20:24:02Z

1

Easier with recursion:

f = n => n ? { val: n % 10, next: f(n / 10 | 0) } : null

console.log( f(807) )

f = (a, b, c = 0) => a && b && { val: (c += a.val + b.val) % 10, 
                                next: f(a.next, b.next, c > 9) } || null

console.log( f( { val: 2, next: { val: 4, next: { val: 3 } } }, 
                { val: 5, next: { val: 6, next: { val: 4 } } } ) )

answered Dec 6, 2017 at 20:24

Slai

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11 Comments

doctopus Over a year ago

what do all the letters represent?

Slai Over a year ago

f = n => n is short for function f(n) { return n }, and I used a and b instead of the l1 and l2 parameters. c is just a temporary variable used to get the carry over +1 if the sum of the 2 digits is more than 9. You can see the translated version on typescriptlang.org/play/…

doctopus Over a year ago

how does n / 10 | 0 work? I just console'd 807 / 10 | 0 and it gave me 80 but no idea how that worked

Slai Over a year ago

@cryptofish123 it's one of the shortcuts for converting a floating point number to integer blog.blakesimpson.co.uk/read/…. Sorry about making the code so cryptic .. I was going for "Leet" code :D

doctopus Over a year ago

Can you explain how c > 9 works? On 3rd iteration c = 10. I assume you want to carry over only 1 to next iteration? How does it do so ?

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