D. Concatenated Multiples(离线处理)

#include<iostream>
#include<cmath>
#include<map>
using namespace std;
#define LL long long
const int maxn=2e5+10;
int  n, m, a[maxn];
map<int, int>ma[12];
LL ans;

int main(){
    cin>>n>>m;
    for(int i=1;i<=n;++i){
        cin>>a[i];
        int x=a[i];
        for(int j=1;j<=10;++j){
            x=1LL*x*10%m;   ma[j][x]++;
        }
    }

    for(int i=1;i<=n;++i){
        int x=a[i], l=0;
        while(x>0){++l; x/=10;}
        if(l==0)l=1;
        ans+=ma[l][(m-a[i]%m)%m];
        x=a[i];
        for(int j=1;j<=l;++j)  x=1LL*x*10%m;
        if((x+a[i])%m==0)--ans;
    }
    cout<<ans<<endl;
}