2. Add Two Numbers

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.

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

答案：

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     struct ListNode *next;
 * };
 */
struct ListNode* addTwoNumbers(struct ListNode* l1, struct ListNode* l2) {
    struct ListNode *l3 = (struct ListNode *)malloc(sizeof(struct ListNode));
    l3->val = 0;
    l3->next = NULL;
    struct ListNode *node = l3;
    int carry = 0;
    while(l1 || l2 || carry) {
        if(l1) {
            carry+=l1->val;
            l1 = l1->next;    
        } 
        if(l2) {
            carry+=l2->val;
            l2 = l2->next;
        }
        node->next = (struct ListNode *)malloc(sizeof(struct ListNode)); 
        node = node->next;
        //必须初始化
        node->next = NULL;

        node->val = carry%10;
        carry /= 10;
    }
    return l3->next;
}