#include<iostream>
#include<cstring>
#include<algorithm>
#include<cmath>
#include<cstdlib>
#include<climits>
#include<stack>
#include<vector>
#include<queue>
#include<set>
#include<bitset>
#include<map>
//#include<regex>
#include<cstdio>
#include <iomanip>
#pragma GCC optimize(2)
#define up(i,a,b) for(int i=a;i<b;i++)
#define dw(i,a,b) for(int i=a;i>b;i--)
#define upd(i,a,b) for(int i=a;i<=b;i++)
#define dwd(i,a,b) for(int i=a;i>=b;i--)
//#define local
typedef long long ll;
typedef unsigned long long ull;
const double esp = 1e-6;
const double pi = acos(-1.0);
const int INF = 0x3f3f3f3f;
const int inf = 1e9;
using namespace std;
ll read()
{
char ch = getchar(); ll x = 0, f = 1;
while (ch<'0' || ch>'9') { if (ch == '-')f = -1; ch = getchar(); }
while (ch >= '0' && ch <= '9') { x = x * 10 + ch - '0'; ch = getchar(); }
return x * f;
}
typedef pair<int, int> pir;
#define lson l,mid,root<<1
#define rson mid+1,r,root<<1|1
#define lrt root<<1
#define rrt root<<1|1
const int N = 1e3 + 10;
const int mod = 1e9 + 7;
ll dp[N], f[N];
int n, nextt[N];
bool vis[N];
ll add(ll a, ll b)
{
ll res = a + b;
if (res >= mod)res -= mod;
return res;
}
int main()
{
n = read();
int pi;
upd(i, 1, n)
{
pi = read();
nextt[i] = pi;
}
dp[1] = 1;
f[1] = 1;
upd(i, 2, n)
{
if (nextt[i] == i)
{
f[i] = 1;
dp[i] = add(dp[i - 1], 2);
}
else {
int j = i;
ll sum = 0;
memset(vis, 0, sizeof(vis));
while (nextt[j] != j)
{
sum++;
vis[j] =1;
j = nextt[j];
}
up(k, j, i)
{
if(!vis[k])
sum = add(sum, f[k]);
sum = add(sum, 1);
}
f[i] = sum;
dp[i] = add(dp[i - 1], sum);
dp[i] = add(dp[i], 1);
}
}
printf("%lld", add(dp[n], 1));
return 0;
}
407B - Long Path
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转载自www.cnblogs.com/LORDXX/p/12599556.html
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