leetcode Add Two Numbers(对指针的一些基本操作)

     ListNode *ptr,*l;
    l = new ListNode(0);//这才是正确的赋值姿势
    ptr = l;//赋给的是地址
    int up = 0,fg1 = 0,fg2 = 0;
    //cout<<"r"<<endl;
    while(1)
    {
        if(fg1 && fg2) break;
        int a,b;
        if(fg1) a = 0;
        else a = l1 -> val;
        if(fg2) b = 0;
        else b = l2 -> val;
       // cout<<a<<" "<<b<<endl;
        l -> next = new ListNode( (a + b + up) % 10);
        if(a + b + up >= 10) up = 1;
        else up = 0;
        if(!(l1 -> next)) fg1 = 1;
        else    l1 = l1 -> next;
        if(!(l2 -> next)) fg2 = 1;
        else    l2 = l2 -> next;
        l = l -> next;
    }
    if(up) l -> next  = new ListNode(up);
        return ptr -> next;