LeetCode Interview Qustions: Add Two Number

/**
* 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) {
// Start typing your C/C++ solution below
// DO NOT write int main() function
ListNode* result = new ListNode(0);
ListNode* p = result;
int carray_bit = 0;
while(l1 && l2) {
p->next = new ListNode(0);
p = p->next;

p->val = l1->val + l2->val + carray_bit;
if (p->val >= 10) {
p->val = p->val%10;
carray_bit = 1;
} else {
carray_bit = 0;
}
l1 = l1->next;
l2 = l2->next;
}

while(l1) {
p->next = new ListNode(0);
p = p->next;

p->val = l1->val + carray_bit;
if (p->val >= 10) {
p->val = p->val%10;
carray_bit = 1;
} else {
carray_bit = 0;
}
l1 = l1->next;
}

while(l2) {
p->next = new ListNode(0);
p = p->next;

p->val = l2->val + carray_bit;
if (p->val >= 10) {
p->val = p->val%10;
carray_bit = 1;
} else {
carray_bit = 0;
}
l2 = l2->next;
}

if (carray_bit) {
p->next = new ListNode(1);
p = p->next;
}
return result->next;
}
};