HDU 5570 balls

代码不是自己写的，这道题看上去是个好题。先囤着。

#include<cstdio>
#include<cstring>
using namespace std;
int n;
int m;
int a[1005][1005];
int sum[1005];
double f[1005][1005];
double g[1005][1005];
double ans;
int main()
{
while(~scanf("%d%d",&n,&m))
{
memset(f,0,sizeof f);
ans = 0;
memset(g,0,sizeof g);
memset(sum,0,sizeof sum);
for(int i=1;i<=n;++i)
{
for(int j = 1;j<=m;++j)
{
scanf("%d",&a[i][j]),sum[i] += a[i][j];
}
}
for(int i=1;i<=n;++i)
{
for(int j=1;j<=m;++j)
{
f[i][j] = f[i-1][j] + 1.0*a[i][j] / sum[i];
g[i][j] = g[i-1][j]*(sum[i]-a[i][j])/sum[i]+(g[i-1][j] + 2*f[i-1][j] + i - 1)*a[i][j]/sum[i];
g[i][j] += 1.0*a[i][j] / sum[i];
}
}
for(int i=1;i<=m;++i)
ans+= g
[i];
printf("%.2lf\n",ans - n * (n-1)/2);
}
return 0;
}