用链表实现大整数加法

// 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) {
ListNode *ret = l1, *pre = l1;
int up = 0;
while (l1 != NULL && l2 != NULL) {
l1->val = l1->val + l2->val + up;
up = l1->val / 10;
l1->val %= 10;
pre = l1;
l1 = l1->next;
l2 = l2->next;
}
if (l1 == NULL)
pre->next = l2;
l1 = pre->next;
while (l1 != NULL) {
l1->val = l1->val + up;
up = l1->val / 10;
l1->val %= 10;
pre = l1;
l1 = l1->next;
}
if (up != 0) {
ListNode *tmp = new ListNode(up);
pre->next = tmp;
}
return ret;
}
};