BZOJ 2720: [Violet 5]列队春游

2720: [Violet 5]列队春游

Time Limit: 5 Sec  Memory Limit: 128 MB
Submit: 189  Solved: 133
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Description

Input

Output

Sample Input

 

Sample Output

 

HINT

Source

interviewstreet--virtual stick

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LTY大佬来嘲讽我了“你水不水，这题还要看题解的？”。

据说$O(N^{3})$的特别好想，优化一下就到了$O(N^{2})$，然后蒟蒻的我就抄了份$O(N)$的题解，然后和LH大爷推了好久。

 #include <cstdio>

 int n, cnt[], sum; double ans;

 signed main(void) {
     scanf("%d", &n); ++n;

     for (int i = , j; i < n; ++i)
         scanf("%d", &j), ++cnt[j];

     for (int i = ; i <= ; ++i)
         ans += 1.0 * cnt[i] * n / (n - sum), sum += cnt[i];

     printf("%.2lf\n", ans);
 }

@Author: YouSiki