Add Two Numbers

    两个十进制数，反向存储在单向链表里，每位数占一个节点。求和的链表。

/**

 * Definition for singly-linked list.

 * struct ListNode {

 *     int val;

 *     ListNode *next;

 *     ListNode(int x) : val(x), next(NULL) {}

 * };

 */

int add(int a, int b, int &carry)

{

    int c = a + b + carry;

    carry = c / 10;

    return c % 10;

}

ListNode *add_two_numbers(ListNode *l1, ListNode *l2) 

{

    ListNode *head = NULL, *rear = NULL;

    if (l1 == NULL) return l2;

    if (l2 == NULL) return l1;

    int carry = 0;

    while (l1 != NULL || l2 != NULL)

    {

        int val = 0;

        if (l1 == NULL)

        {

            val = add(0, l2->val, carry);

            l2 = l2->next;

        }

        else if (l2 == NULL)

        {

            val = add(l1->val, 0, carry);

            l1 = l1->next;

        }

        else

        {

            val = add(l1->val, l2->val, carry);

            l1 = l1->next;

            l2 = l2->next;

        }

        ListNode *new_node = new ListNode(val);

        if (head == NULL) head = new_node;

        if (rear == NULL)

        {

            rear = new_node;

        }

        else

        {

            rear->next = new_node;

            rear = rear->next;

        }

    }

    if (carry != 0 && rear != NULL)

    {

        rear->next = new ListNode(carry);

    }

    return head;

}