CodeForces 340E Iahub and Permutations 错排dp

#include<bits/stdc++.h>
using namespace std;
#define Fopen freopen("_in.txt","r",stdin); freopen("_out.txt","w",stdout);
#define LL long long
#define ULL unsigned LL
#define fi first
#define se second
#define pb push_back
#define lson l,m,rt<<1
#define rson m+1,r,rt<<1|1
#define lch(x) tr[x].son[0]
#define rch(x) tr[x].son[1]
#define max3(a,b,c) max(a,max(b,c))
#define min3(a,b,c) min(a,min(b,c))
typedef pair<int,int> pll;
const int inf = 0x3f3f3f3f;
const int _inf = 0xc0c0c0c0;
const LL INF = 0x3f3f3f3f3f3f3f3f;
const LL _INF = 0xc0c0c0c0c0c0c0c0;
const LL mod =  (int)1e9+7;
const int N = 1e5 + 100;
LL dp[N];
int fvis[N];
int vis[N];
int cnt1, cnt2;
int main(){
    int n, t;
    scanf("%d", &n);
    for(int i = 1; i <= n; ++i){
        scanf("%d", &t);
        if(t == -1) continue;
        vis[t] = 1;
        fvis[i] = 1;
    }
    for(int i = 1; i <= n; ++i){
        if(vis[i] + fvis[i] == 2) ;
        else if(!vis[i] && fvis[i]) cnt1++;
        else if(vis[i] + fvis[i] == 0) cnt2++;
    }
    dp[0] = 1;
//    cout << cnt1 << " " << cnt2 << endl;
    for(int i = 1; i <= cnt1; ++i) dp[0] = dp[0] * i % mod;
    for(int i = 1; i <= cnt2; ++i){
        dp[i] = cnt1 * dp[i-1] % mod + (i-1) * dp[i-1] % mod;
        if(i >= 2) dp[i] += (i-1) * dp[i-2] % mod;
        dp[i] %= mod;
//        cout << dp[i] << endl;
    }
    cout << dp[cnt2] << endl;
    return 0;
}