一个循环内从左向右加，当l1==NULL && l2==NULL 退出，如果有进位额外申请一个节点保存进位：

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     struct ListNode *next;
 * };
 */
struct ListNode* addTwoNumbers(struct ListNode* l1, struct ListNode* l2) {

    struct ListNode *dummyhead = malloc(sizeof(struct ListNode));
    dummyhead->next = NULL;
    int val, carry = 0;
    struct ListNode *node ,*last;
    last = dummyhead;
    while(1){
        if(l1&&l2){
            val = (l1->val+l2->val+carry)%10;
            carry = (l1->val+l2->val+carry)/10;
            l1 = l1->next;
            l2 = l2->next;
        }else if(l1 != NULL && l2 == NULL){
            val = (l1->val + carry)%10;
            carry = (l1->val+carry)/10;
            l1 = l1->next;
        }else if(l1 == NULL && l2 != NULL){
            val = (l2->val + carry)%10;
            carry = (l2->val+carry)/10;
            l2 = l2->next;
        }else
            break;

        node = malloc(sizeof(struct ListNode));
        node->val = val;
        node->next = NULL;
        last->next = node;
        last = last->next;
    }

    if(carry){
        node = malloc(sizeof(struct ListNode));
        node->val = carry;
        node->next = NULL;
        last->next = node;  
    }
    
    return dummyhead->next;
    
}