BZOJ 1419 Red is good

期望DP

f[i][j]表示i张红牌，j张黑牌

从 f[i-1][j]，f[i][j-1]，0 转移过来，注意下界为0要判，因为取成负数肯定不优

#include<cstdio>#include<cstring>#include<algorithm>#define N 5005using namespace std;double f[2][N];int main(){    int n, m, cur=0;    scanf("%d%d",&n,&m);    for(int i = 1; i <= n; i++,cur^=1)    {        f[cur][0]=i;        for(int j = 1; j <= m; j++)            f[cur][j]=max(0.0 , 1.0*i/(i+j)*(f[cur^1][j]+1) + 1.0*j/(i+j)*(f[cur][j-1]-1));    }    printf("%lf\n",f[cur^1][m]-0.0000005);}