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ID: 2; medium

## Solution 1 (Go)

/**
* type ListNode struct {
* Val int
* Next *ListNode
* }
*/
func addTwoNumbers(l1 *ListNode, l2 *ListNode) *ListNode {
head := &ListNode{Val: 0} // dummy node
val1, val2, carry, runner := 0, 0, 0, head
for l1 != nil || l2 != nil || carry == 1 {
if l1 == nil {
val1 = 0
} else {
val1 = l1.Val
l1 = l1.Next
}
if l2 == nil {
val2 = 0
} else {
val2 = l2.Val
l2 = l2.Next
}
runner.Next = &ListNode{Val: (val1 + val2 + carry) % 10}
carry = (val1 + val2 + carry) / 10
runner = runner.Next
}
}

## Solution 2 (Java)

/**
* public class ListNode {
* int val;
* ListNode next;
* ListNode() {}
* ListNode(int val) { this.val = val; }
* ListNode(int val, ListNode next) { this.val = val; this.next = next; }
* }
*/
class Solution {
public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
ListNode dummy = new ListNode(0);
int carry = 0;
while (l1 != null && l2 != null) {
int val = (l1.val + l2.val + carry) % 10;
carry = (l1.val + l2.val + carry) / 10;
ListNode newNode = new ListNode(val);
l1 = l1.next;
l2 = l2.next;
}
while (l1 == null && l2 != null) {
int val = (l2.val + carry) % 10;
carry = (l2.val + carry) / 10;
ListNode newNode = new ListNode(val);
l2 = l2.next;
}
while (l2 == null && l1 != null) {
int val = (l1.val + carry) % 10;
carry = (l1.val + carry) / 10;
ListNode newNode = new ListNode(val);
l1 = l1.next;
}
if (carry == 1) head.next = new ListNode(1);
return dummy.next;
}
}
// 9 -> 9 -> 9 -> 9
// 9 -> 9
// 8 -> 9 -> 0 -> 0 -> 1

## Solution 3 (Java)

/**
* public class ListNode {
* int val;
* ListNode next;
* ListNode() {}
* ListNode(int val) { this.val = val; }
* ListNode(int val, ListNode next) { this.val = val; this.next = next; }
* }
*/
class Solution {
public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
ListNode dummy = new ListNode(0);
ListNode curr = dummy;
int carry = 0;
while (l1 != null || l2 != null || carry != 0) {
int x = (l1 != null) ? l1.val : 0;
int y = (l2 != null) ? l2.val : 0;
int sum = x + y + carry;
carry = sum / 10;
curr.next = new ListNode(sum % 10);
curr = curr.next;
if (l1 != null) l1 = l1.next;
if (l2 != null) l2 = l2.next;
}
return dummy.next;
}
}
// 1111111
// 9999999
// 9999
// 10009998