Codeforces sort B basketball team simple probability

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Question:

Given N M H

Indicates that there are m departments, and an individual is in the department H

The following M numbers indicate the number of people in each department. (Including himself)

N people are randomly selected among these people, and the probability of a person in the same department as this person is asked.

This person must be among the N people picked out.

Reverse Thinking. The answer is 1-impossible probability.

Impossible probability = C (n-1, sum-1-a [H])/C (n-1, Sum-1)

The denominator of the two combinations is the same, so you only need to divide the molecular parts of the two combinations.

#include <cstdio>#include <iostream>#include <algorithm>#include <string.h>#include <map>#include <set>using namespace std;#define N 10010int n, m, h, a[N];void solve(){    int sum = 0;    for(int i = 1; i <= m; i++) scanf("%d",&a[i]), sum += a[i];    if(sum < n){        puts("-1");return ;    }    n--;    sum--; a[h]--;    if(sum - a[h] < n){puts("1");return;}    double ans = 1.0;    double x = sum-a[h], y = sum;    for(int i = 1; i <= n; i++) {        ans *= x / y;        x--; y--;    }    printf("%.10f\n", 1.0 - ans);}int main(){    while(~scanf("%d %d %d",&n,&m,&h)){        solve();    }    return 0;}

Codeforces sort B basketball team simple probability