组合概率

#include<iostream>

using namespace std;

double dp[110][11000];

int main()
{
if(freopen("/Users/richard/ClionProjects/Didi/in","r",stdin) == NULL) {
cout << "failed open!" << endl;
}

int n,a,b,x;
double p;
cin >> n >> a >> b >> x;

memset(dp,0, sizeof(dp));

p = 1.0f/(b-a+1);

for(int i = 1; i <= n; ++i) {
for(int j = a *i; j <= b*i && j <= x; ++j) {//i个数，其和的range为a*i~b*i
if(i == 1) {
dp[i][j] = p;
} else {
double pp = 0;
for(int z = a; z <= b; ++z) {
if(j >= z)
pp += dp[i-1][j-z]*p;//动态规划dp
}
dp[i][j] = pp;
}
}
}

printf("%0.4f\n", dp
[x]);

return 0;
}