2. Add Two Numbers

https://leetcode.com/problems/add-two-numbers/

JavaScript

/**
 * Definition for singly-linked list.
 * function ListNode(val) {
 *     this.val = val;
 *     this.next = null;
 * }
 */
/**
 * @param {ListNode} l1
 * @param {ListNode} l2
 * @return {ListNode}
 */
var addTwoNumbers = function(l1, l2) {
  var carry = 0
  var head
  var current
  head = current = new ListNode(null)

  while (l1 || l2) {
    var sum = carry

    if (l1) {
      sum += l1.val
      l1 = l1.next
    }
    if (l2) {
      sum += l2.val
      l2 = l2.next
    }

    carry = (sum / 10) >> 0
    current.next = new ListNode(sum % 10)
    current = current.next
  }

  if (carry) {
    current.next = new ListNode(carry)
  }

  return head.next
}

Python

# Definition for singly-linked list.
# class ListNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.next = None


class Solution(object):

    def addTwoNumbers(self, l1, l2):
        """
        :type l1: ListNode
        :type l2: ListNode
        :rtype: ListNode
        """
        head = current = ListNode(None)
        carry = 0

        while l1 is not None or l2 is not None:
            sum = carry

            if l1 is not None:
                sum += l1.val
                l1 = l1.next
            if l2 is not None:
                sum += l2.val
                l2 = l2.next

            carry = sum // 10
            current.next = ListNode(sum % 10)
            current = current.next

        if carry:
            current.next = ListNode(carry)

        return head.next

Go

package main

type ListNode struct {
	Val  int
	Next *ListNode
}

func addTwoNumbers(l1 *ListNode, l2 *ListNode) *ListNode {
	carry := 0
	head := &ListNode{}
	current := head

	for l1 != nil || l2 != nil {
		sum := carry

		if l1 != nil {
			sum += l1.Val
			l1 = l1.Next
		}

		if l2 != nil {
			sum += l2.Val
			l2 = l2.Next
		}

		carry = sum / 10
		current.Next = &ListNode{Val: sum % 10}
		current = current.Next
	}

	if carry > 0 {
		current.Next = &ListNode{Val: carry}
	}

	return head.Next
}