cf 398B. Painting The Wall

23333，还是不会。。%%%http://hzwer.com/6276.html

#include <bits/stdc++.h>
#define LL long long
#define lowbit(x) x&(-x)
#define inf 0x3f3f3f3f
using namespace std;
inline int ra()
{
    int x=0,f=1; char ch=getchar();
    while (ch<'0' || ch>'9') {if (ch=='-') f=-1; ch=getchar();}
    while (ch>='0' && ch<='9') {x=x*10+ch-'0'; ch=getchar();}
    return x*f;
}
double dp[2005][2005];
int n,m,t1,t2;
bool r[2005],c[2005];
int main(int argc, char const *argv[])
{
    n=ra(); m=ra();
    t1=t2=n;
    for (int i=1; i<=m; i++)
    {
        int x=ra(),y=ra();
        if (!r[x]) r[x]=1,t1--;
        if (!c[y]) c[y]=1,t2--;
    }
    for (int i=1; i<=n; i++)
    {
        dp[i][0]=dp[i-1][0]+(double)n/i;
        dp[0][i]=dp[0][i-1]+(double)n/i;
    }
    for (int i=1; i<=t1; i++)
        for (int j=1; j<=t2; j++)
        {
            dp[i][j]=n*n;
            dp[i][j]+=i*j*dp[i-1][j-1];
            dp[i][j]+=i*(n-j)*dp[i-1][j];
            dp[i][j]+=j*(n-i)*dp[i][j-1];
            dp[i][j]/=(n*n-(n-i)*(n-j));
        }
    printf("%.10lf
",dp[t1][t2]);
    return 0;
}