https://ac.nowcoder.com/acm/contest/3674/I 不是太能理解题解里给出的D[n]=(n-1)*(D[n-1]+D[n-2]) 另一种写法: #include <bits/stdc++.h> using namespace std; const int Mod=1e9+7; int main() { int n; string s; cin>>n; long long a[n+5]; getline(cin,s); a[0]=0,a[1]=1; for(i...