Problem Description

You are given two non-empty linked lists representing two non-negative integers. The digits are stored in reverse order and each of their nodes contains a single digit. Add the two numbers and return it as a linked list.

You may assume the two numbers do not contain any leading zero, except the number 0 itself.

Example

Input: (2 -> 4 -> 3) + (5 -> 6 -> 4)
Output: 7 -> 0 -> 8
Explanation: 342 + 465 = 807.

Java:

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        ListNode res =  new ListNode(-1);
        ListNode cur = res;
        int carry = 0;
        
        while (l1 != null || l2 != null) {
            carry = carry + (l1 == null ? 0 : l1.val) + (l2 == null ? 0 : l2.val);
            l1 = l1 == null ? null : l1.next;
            l2 = l2 == null ? null : l2.next;
            cur.next = new ListNode(carry % 10);
            cur = cur.next;
            carry /= 10;
        }
        
        if (carry != 0) {
            cur.next = new ListNode(carry);
        }
        
        return res.next;
    }
}

Python:

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

class Solution:
    
    carry = 0
    
    def addTwoNumbers(self, l1, l2):
        """
        :type l1: ListNode
        :type l2: ListNode
        :rtype: ListNode
        """
        if l1 is None and l2 is None:
            return None if self.carry == 0 else ListNode(self.carry)
        else:
            self.carry += (0 if l1 is None else l1.val) + (0 if l2 is None else l2.val)
            res = ListNode(self.carry % 10)
            self.carry //= 10
            res.next = self.addTwoNumbers(None if l1 is None else l1.next, None if l2 is None else l2.next)
            return res

Go:

/**
 * Definition for singly-linked list.
 * type ListNode struct {
 *     Val int
 *     Next *ListNode
 * }
 */
func addTwoNumbers(l1 *ListNode, l2 *ListNode) *ListNode {
    var res *ListNode = new(ListNode) // var res *ListNode = &ListNode{0, nil}
    cur := res
    carry := 0
    
    for l1 != nil || l2 != nil || carry != 0 {
        if l1 != nil {
            carry += l1.Val
            l1 = l1.Next
        }
        
        if l2 != nil {
            carry += l2.Val
            l2 = l2.Next
        }
        
        cur.Next = new(ListNode)
        cur = cur.Next
        cur.Val = carry % 10
        carry /= 10
    }
    
    return res.Next
}