打气球(记忆化dp+数学期望)

过于菜QAQ

#include <cstdio>
#include <iostream>
#include <iomanip>
#include <cstring>
#include <cmath>
#include <cstdlib>
#include <algorithm>
using namespace std;
inline int R(){
    char t=getchar();int o=0;bool F=0;
    while(t<48||t>57)F|=t==45,t=getchar();
    for(;t<58&&t>47;t=getchar())o=(o<<1)+(o<<3)+t-48;
    return F?-o:o;
}
bool h[2010], s[2010];
int n;
double f[2010][2010];
int m;

double dp( int x, int y ){
    if( f[x][y] != -1 ) return f[x][y];
    if( ! x && ! y ) return f[x][y] = 0;
    double ans = 0;
    if( x ) ans += dp( x-1, y ) * x * ( n - y );
    if( y ) ans += dp( x, y-1 ) * ( n - x ) * y;
    if( x && y ) ans += dp( x-1, y-1 ) * x * y;
    return f[x][y] = (double) ( ans + (double) n * n ) / ( (double) n * ( x + y ) - x * y ) ;
}
int main()
{
//  freopen("data4.in","r",stdin);
//  freopen("shoot.out","w",stdout);
    n = R(); m = R(); int a, b;
    for( int i = 1; i <= m; i ++ ){
        a = R(); b = R();
        h[a] = 1; s[b] = 1;
    }
    int c = 0, r = 0;
    for( int i = 1; i <= n; i ++ ){
        if( ! h[i] ) c ++;
        if( ! s[i] ) r ++;
    }

    for( int i = 0; i <= n; i ++ )
        for( int j = 0; j <= n; j ++ )
            f[i][j] = -1;

    printf("%.2lf",dp(c,r));
    return 0;
}