# Question

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 contain 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.
```

# Solution

Iterate through the linked list and add digits by digits.

```
# 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
"""
carry = 0
head = None
prev = None
while l1 is not None or l2 is not None:
x = l1.val if l1 is not None else 0
y = l2.val if l2 is not None else 0
sum = x + y + carry
if sum >= 10:
carry = 1
res = sum - 10
else:
carry = 0
res = sum
node = ListNode(res)
if head is None:
head = node
if prev is not None:
prev.next = node
if l1 is not None:
l1 = l1.next
if l2 is not None:
l2 = l2.next
prev = node
if carry == 1:
node = ListNode(carry)
prev.next = node
return head
```

# Follow Up: Add Two Numbers II

You are given two **non-empty** linked lists representing two non-negative integers. The most significant digit comes first and each of their nodes contain 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.

**Follow up:**
What if you cannot modify the input lists? In other words, reversing the lists is not allowed.

Example:

```
Input: (7 -> 2 -> 4 -> 3) + (5 -> 6 -> 4)
Output: 7 -> 8 -> 0 -> 7
```

# Solution

Convert the linked list to int value, sum them, and convert the sum to string to get each digit by iterating through the string.

```
# 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
"""
x = self.listToInt(l1)
y = self.listToInt(l2)
res = str(x + y)
head = None
prev = None
for c in res:
node = ListNode(int(c))
if head is None:
head = node
if prev is not None:
prev.next = node
prev = node
return head
def listToInt(self, l):
x = 0
while True:
x *= 10
x += l.val
if l.next == None:
break
else:
l = l.next
return x
```