[LeetCode]Add Two Numbers

题目描述：

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.

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

解题方案：

题目意思很简单，但是我调试了半天，并且提交了很多次，拉低了整体AC率。下面是该题代码：

class Solution {
public:
    ListNode *addTwoNumbers(ListNode *l1, ListNode *l2) {
        int carry = 0;

        struct ListNode *head1 = l1;
        struct ListNode *head2 = l2;
        struct ListNode *pre = NULL;

        int count1 = 0;
        int count2 = 0;
        while (head1 != NULL) { ++count1; head1 = head1->next;}
        while (head2 != NULL) { ++count2; head2 = head2->next;}
        if (count1 < count2) {
            struct ListNode *temp = l1;
            l1 = l2;
            l2 = temp;
        }

        head1 = l1;
        head2 = l2;

        while (true) {
            int temp = head1->val + head2->val;
            head1->val = (temp + carry) % 10;
            carry = (temp + carry) / 10;

            pre   = head1;
            head1 = head1->next;
            head2 = head2->next;
            if (head2 == NULL) {
                break;
            }
        }

        while(head1 != NULL) {
            int temp = head1->val;
            head1->val = (temp + carry) % 10;
            carry = (temp + carry) / 10;
            pre = head1;
            head1 = head1->next;
        }

        if (carry != 0) {
            struct ListNode *last = new ListNode(carry);
            pre->next = last;
        }
        return l1;
    }
};