题解 | #附加题#
附加题
http://www.nowcoder.com/practice/58b04ed2865f4ff4921290f1bd4ee486
#include<stdio.h>
#include<vector>
using namespace std;
const int mod = 1000000007;
int main(){
int n, i;
scanf("%d", &n);
vector<unsigned int> p(n+1), dp(n+2);
for (i = 1; i <= n; ++i) scanf("%d", &p[i]);
dp[1] = 0;
for (i = 1; i <= n; ++i) {
dp[i+1] = (2*dp[i] - dp[p[i]] + 2 + mod) % mod;
}
printf("%d", dp[n+1]);
return 0;
}
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