Cod sursă (job #786116)

// Author: Tanasescu Andrei-Rares
#include <iostream>
#include <fstream>

using namespace std;

ifstream fin ("dejavu.in");
ofstream fout ("dejavu.out");

typedef long long ll;

const int Nmax=5e3+5, inf=1e9+5, MOD=1e9+7;

int n, q;

// dp[i][j] = modalitatea de a continua un prefix ce are i litere singuratice
//            si j perechi de litere
// lungimea prefixului = i+j*2
int dp[Nmax][Nmax/2];
int fr[Nmax], aib1[Nmax], aib2[Nmax];

inline void update(int aib[], int pos, int val){
    while (pos<Nmax){
        aib[pos]+=val;
        pos+=pos&-pos;
    }
}
inline int query(int aib[], int pos){
    int sum=0;
    while (pos>0){
        sum+=aib[pos];
        pos-=pos&-pos;
    }
    return sum;
}
inline void clr(int aib[]){
    for (int i=1; i<Nmax; i++)
        aib[i]=0;
}

int main()
{
    fin>>n>>q;

    // initializare dp
    for (int j=0; j*2<=n; j++){
        int i=n-j*2;
        dp[i][j]=1;
    }
    // build dp 
    for (int l=n-1; l>=0; l--)
        for (int j=0; j*2<=l; j++){
            int i=l-j*2;
            dp[i][j]=(ll)dp[i+1][j]*(n-i-j)%MOD;
            if (i!=0)
                dp[i][j]=(dp[i][j]+(ll)dp[i-1][j+1]*i)%MOD;
        }

    // answer queries
    while (q--){
        int nr, cnt1=0, cnt2=0, crt1, crt0;
        ll sol=0;
        for (int i=0; i<n; i++){
            fin>>nr;
            crt1=query(aib1, nr-1);
            crt0=nr-1-crt1-query(aib2, nr-1);

            sol=(sol+(ll)crt0*dp[cnt1+1][cnt2])%MOD;
            if (cnt1!=0)
                sol=(sol+(ll)crt1*dp[cnt1-1][cnt2+1])%MOD;

            fr[nr]++;
            if (fr[nr]==1){
                cnt1++;
                update(aib1, nr, 1);
            }
            else{
                cnt2++;
                cnt1--;
                update(aib1, nr, -1);
                update(aib2, nr, 1);
            }
        }
        fout<<sol<<'\n';

        for (int i=1; i<=n; i++)
            fr[i]=0;
        clr(aib1);
        clr(aib2);

    }

    return 0;
}