计算 $[1,n]$ 所有整数的每一位数字之和

例如：sumDigitsUpTo(11) = 1+2+3+4+5+6+7+8+9+(1+0)+(1+1)

ll sumDigitsUpTo(ull n){if (n<=0) return 0;ll res = 0;for (ll pos=1;pos<=n;pos*=10){ll higher = n/(pos*10);ll cur = (n/pos)%10;ll lower = n%pos;res += higher*45*pos;res += cur*(lower+1);res += (cur*(cur-1)/2)*pos;	}return res;
}