poj3744--Scout YYF I(概率dp第五弹：矩阵优化)

#include <cstdio>
#include <cstring>
#include <algorithm>
using namespace std;
struct node
{
    double x , y ;
    double  a[2][2] ;
} s , k ;
node f(node p,node q)
{
    node k ;
    int i , j , l ;
    k.x = p.x ;
    k.y = q.y ;
    for(i = 0 ; i < k.x ; i++)
    {
        for(j = 0 ; j < k.y ; j++)
        {
            k.a[i][j] = 0 ;
            for(l = 0 ; l < p.y ; l++)
                k.a[i][j] += p.a[i][l]*q.a[l][j] ;
        }
    }
    return k ;
}
node pow(node p,int m)
{
    node q ;
    q.x = q.y = 2 ;
    q.a[0][0] = q.a[1][1] = 1 ;
    q.a[0][1] = q.a[1][0] = 0 ;
    if(m == 0)
        return q ;
    q = pow(p,m/2);
    q = f(q,q);
    if( m%2 )
        q = f(q,p);
    return q;
}
int main()
{
    int n , i , j , a[12] , m , flag;
    double p ;
    while(scanf("%d %lf", &n, &p)!=EOF)
    {
        flag = a[0] = 0 ;
        for(i = 1 ; i <= n ; i++)
        {
            scanf("%d", &a[i]);
            if(a[i] == 1) flag = 1 ;
        }
        if(flag)
        {
            printf("0.0000000\n");
            continue ;
        }
        sort(a,a+n+1);
        s.x = 2 ;
        s.y = 1 ;
        s.a[0][0] = 1 ;
        k.x = k.y = 2 ;
        k.a[0][0] = p ;
        k.a[0][1] = (1-p) ;
        k.a[1][0] = 1 ;
        k.a[1][1] = 0 ;
        for(i = n-1 ; i >= 0 ; i--)
        {
            s.a[1][0] = s.a[0][0] ;
            s.a[0][0] = 0 ;
            m = ( a[i+1]-1-a[i] ) ;
            s = f(pow(k,m),s);
        }
        printf("%.7lf\n", s.a[0][0]);
    }
    return 0;
}