Little Sub and Mr.Potato's Math Problem-构造

#include<bits/stdc++.h>
using namespace std;
#define ll long long
ll t,n,k,m,po[222];
ll getmin(ll x)
{
    ll ret=0,data[55],id=0;
    po[0]=1;
    while(x)
    {
        data[id++]=x;
        x/=10;
        po[id]=po[id-1]*10;
    }
    for(int i=0; i<id; i++)
        ret+=data[i]-po[id-i-1]+1;
    return ret;
}
int main()
{
    scanf("%lld",&t);
    while(t--)
    {
        scanf("%lld%lld",&k,&m);
        m-=getmin(k);
        if(m<0)printf("0\n");
        else if(m==0)printf("%lld\n",k);
        else
        {
            ll hig,big;
            for(ll i=1;; i*=10)
                if(k/i<10)
                {
                    big=i;
                    hig=k-i;
                    break;
                }
            if(big==k)printf("0\n");
            else
            {
                big*=10;
                hig*=10;
                while(m>hig)
                {
                    m-=hig;
                    hig*=10;
                    big*=10;
                }
                printf("%lld\n",big+m-1);
            }
        }
    }
    return 0;
}