17国庆day2

Posted 2020-10-09 yijiull

 

Tournament Wins

 Gym - 101201K 

题意: 2^k个人,你排名第r, 问你期望能赢的次数.

至少赢 i 次的概率是 C(2^i-1, 2^k-r) / C(2^i-1, 2^k-1) ,化简一下

 1 #include <bits/stdc++.h>
 2 using namespace std;
 3 int k, r;
 4 
 5 int main() {
 6         scanf("%d %d", &k, &r);
 7         int m = (1<<k) - r;
 8         double ans = 0;
 9         int i;
10         for(i = 1; (1<<i)-1 <= m; i++){
11             double p = 1.0;
12            // int a = (1<<k) - (1<<i) + 2 - r;
13             //int c = (1<<k) - r + 1;
14             int a = (1<<k) - 1;
15             int c = (1<<k) - r;
16             for(int j = 0; j <(1<<i)-1 ; j++) {
17                 p = p*c/a;
18                 a--;
19                 c--;
20             }
21             ans += p;
22         }
23         printf("%.5lf\n", ans);
24 }

View Code