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  1. #include <cstdio>
  2. #include <cmath>
  3. #include <map>
  4. #include <cstring>
  5. #include <algorithm>
  6. using namespace std;
  7. #define mst(a,b) memset((a),(b),sizeof(a))
  8. #define rush() int T;scanf("%d",&T);while(T--)
  9. typedef long long ll;
  10. const int maxn = 1000005;
  11. const ll mod = 1e9+7;
  12. const int INF = 0x3f3f3f3f;
  13. const double eps = 1e-6;
  14. int a[maxn],pre[maxn];
  15. int main()
  16. {
  17.     int n,x;
  18.     while(~scanf("%d",&n))
  19.     {
  20.         mst(pre,0);
  21.         ll ans=0;
  22.         ll temp=0;
  23.         for(int i=1;i<=n;i++)
  24.         {
  25.             scanf("%d",&x);
  26.             temp+=i-pre[x];
  27.             pre[x]=i;
  28.             ans+=(ll)temp*2-1;
  29.         }
  30.         printf("%.6f\n",ans*1.0/n/n);
  31.     }
  32.     return 0;
  33. }
Success #stdin #stdout 0s 4296KB
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https://ideone.com/VwKchs
language:
C++14 (gcc 8.3)
created:
5 years ago
visibility:
public

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