Let’s start with a very classical problem. Given an array a[1…n] of positive numbers, if the value of each element in the array is distinct, how to find the maximum element in this array? You may write down the following pseudo code to solve this problem:
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function find_max(a[1…n])
max=0;
for each v from a
if(max<v)
max=v;
return max;
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However, our problem would not be so easy. As we know, the sentence ‘max=v’ would be executed when and only when a larger element is found while we traverse the array. You may easily count the number of execution of the sentence ‘max=v’ for a given array a[1…n].
Now, this is your task. For all permutations of a[1…n], including a[1…n] itself, please calculate the total number of the execution of the sentence ‘max=v’. For example, for the array [1, 2, 3], all its permutations are [1, 2, 3], [1, 3, 2], [2, 1, 3], [2, 3, 1], [3, 1, 2] and [3, 2, 1]. For the six permutations, the sentence ‘max=v’ needs to be executed 3, 2, 2, 2, 1 and 1 times respectively. So the total number would be 3+2+2+2+1+1=11 times.
Also, you may need to compute that how many times the sentence ‘max=v’ are expected to be executed when an array a[1…n] is given (Note that all the elements in the array is positive and distinct). When n equals to 3, the number should be 11/6= 1.833333.
Input
The first line of the input contains an integer T(T≤100,000), indicating the number of test cases. In each line of the following T lines, there is a single integer n(n≤1,000,000) representing the length of the array.
Output
For each test case, print a line containing the test case number (beginning with 1), the total number mod 1,000,000,007
and the expected number with 6 digits of precision, round half up in a single line.
Sample Input
2 2 3
Sample Output
Case 1: 3 1.500000 Case 2: 11 1.833333
思路；第n項的交換次數為F[n]=(n-1)!+F[n-1]*n;后面的為res[n]=1.0/n+res[n-1];
預處理一下輸出就行了
代碼：
#include<cstdio> #include<iostream> #include<cstring> #include<algorithm> #include<queue> #include<stack> #include<set> #include<map> #include<vector> #include<cmath>const int maxn=1e5+5; const int mod=1e9+7; typedef long long ll; using namespace std; ll f[10*maxn]; double res[10*maxn]; int main() {ll a=1;f[0]=0;f[1]=1;res[1]=1;for(int t=2;t<=1000000;t++){f[t]=((a*(t-1))%mod+((t)*f[t-1])%mod)%mod;a=(a*(t-1))%mod;res[t]=1.0/t+res[t-1];//printf("%.6f\n",res[t]); }int T;int n;cin>>T;int cnt=1;while(T--){scanf("%d",&n);printf("Case %d: %d ",cnt++,f[n]);printf("%.6f\n",res[n]);}return 0; }
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