hdu4405--Aeroplane chess(求期望)

分享一篇求期望的文章 http://kicd.blog.163.com/blog/static/126961911200910168335852/

从后往回推求期望

//Accepted    4405    234MS    8248K    757 B    Java
import java.util.Scanner;

public class Main {

    public static void main(String[] args) {
        
        Scanner sc = new Scanner(System.in);
        
        while(sc.hasNext()){
            
            int n = sc.nextInt();
            int m = sc.nextInt();
            if(n == 0 && m == 0) break;
            double e[] = new double[n+1];
            int useif[] = new int[n+1];
            for(int i = 0; i < m ; ++i){
                int a = sc.nextInt();
                int b = sc.nextInt();
                useif[a] = b;
            }
            e[n] = 0;
            for(int i = n - 1; i >= 0 ; --i){
                if(useif[i] > 0){
                    e[i] = e[useif[i]];
                    continue;
                }
                e[i] = 1;
                for(int j = 1; j < 7; j++){
                    if(i + j > n) break;
                    else
                        e[i] += e[i+j]/6;
                }
            }
            System.out.printf("%.4f",e[0]);
            System.out.println();
        }
    }
    
}