2. Add Two Numbers

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */

static int wing=[]()
{
    std::ios::sync_with_stdio(false);
    cin.tie(NULL);
    return 0;
}();

class Solution 
{
public:
    ListNode* addTwoNumbers(ListNode* l1, ListNode* l2) 
    {
        if(l1==NULL)
            return l2;
        if(l2==NULL)
            return l1;
        int flag=0;
        ListNode *head=new ListNode(0);
        ListNode *res=head;
        while(l1||l2)
        {
            int vl1,vl2;
            vl1= l1==NULL? 0:l1->val;
            vl2= l2==NULL? 0:l2->val;
            int sum=vl1+vl2+flag;
            flag=sum/10;
            head->next=new ListNode(sum%10);
            head=head->next;
            if(l1)
                l1=l1->next;
            if(l2)
                l2=l2->next;
        }
        if(flag)
            head->next=new ListNode(flag);
        return res->next;   
    }
};

使用新建链表的方法较为简单