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

IUNPUT: (2 -> 4 -> 3) + (5 -> 6 -> 4) (i.e. 342 + 465)

OUTPUT: 7 -> 0 -> 8 (i.e. 342 + 465 = 807)

**Thoughts:**

We just add element by element, as the way we manually do addition. We need to take care of two things:

- We should not forge the carry, especially when we have one more digit just for the carry. (e.g., 5 + 7 = 12, the ‘1’ in 12 is due to the carry from last digit)
- We should be careful about the fact that these two numbers might not be of the same digits. (e.g., 7 + 3456)

**Code (Java):**

/**
* Definition for singly-linked list.
* public class ListNode {
* int val;
* ListNode next;
* ListNode(int x) {
* val = x;
* next = null;
* }
* }
*/
public class Solution {
public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
int carry = 0;
ListNode l3 = null;
ListNode iter = null;
while(l1 != null || l2 != null) {
int v1 = l1 == null ? 0 : l1.val;
int v2 = l2 == null ? 0 : l2.val;
int sum = (v1 + v2 + carry) % 10;
carry = (v1 + v2 + carry) / 10;
ListNode node = new ListNode(sum);
node.next = null;
if(l3 == null) {
iter = node;
l3 = node;
} else {
iter.next = node;
iter = node;
}
l1 = l1 == null? null : l1.next;
l2 = l2 == null? null : l2.next;
}
if(carry != 0) {
ListNode node = new ListNode(carry);
node.next = null;
iter.next = node;
iter = node;
}
return l3;
}
}

**Code (C++):**

/**
* Definition for singly-linked list.
* struct ListNode {
* int val;
* ListNode *next;
* ListNode(int x) : val(x), next(NULL) {}
* };
*/
class Solution {
public:
ListNode *addTwoNumbers(ListNode *l1, ListNode *l2) {
int carry = 0;
ListNode *l3 = NULL;
ListNode *iter = NULL;
while(l1 != NULL || l2 != NULL) {
int v1 = l1 == NULL ? 0 : l1->val;
int v2 = l2 == NULL ? 0 : l2->val;
int sum = (v1 + v2 + carry) % 10;
carry = (v1 + v2 + carry) / 10;
ListNode *node = new ListNode(sum);
if(l3 == NULL) {
iter = node;
l3 = node;
} else {
iter->next = node;
iter = node;
}
l1 = l1 == NULL? NULL : l1->next;
l2 = l2 == NULL? NULL : l2->next;
}
if(carry != 0) {
ListNode *node = new ListNode(carry);
node->next = NULL;
iter->next = node;
iter = node;
}
return l3;
}
};

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