HDU 4405 Aeroplane chess(概率DP)

Aeroplane chess

Time Limit: 2000/1000 MS (Java/Others) Memory Limit: 32768/32768 K (Java/Others)
Total Submission(s): 4033 Accepted Submission(s): 2577

Problem Description
Hzz loves aeroplane chess very much. The chess map contains N+1 grids labeled from 0 to N. Hzz starts at grid 0. For each step he throws a dice(a dice have six faces with equal probability to face up and the numbers on the faces are 1,2,3,4,5,6). When Hzz is at grid i and the dice number is x, he will moves to grid i+x. Hzz finishes the game when i+x is equal to or greater than N.

There are also M flight lines on the chess map. The i-th flight line can help Hzz fly from grid Xi to Yi (0<Xi<Yi<=N) without throwing the dice. If there is another flight line from Yi, Hzz can take the flight line continuously. It is granted that there is no two or more flight lines start from the same grid.
Please help Hzz calculate the expected dice throwing times to finish the game.

Input
There are multiple test cases.
Each test case contains several lines.
The first line contains two integers N(1≤N≤100000) and M(0≤M≤1000).
Then M lines follow, each line contains two integers Xi,Yi(1≤Xi<Yi≤N).
The input end with N=0, M=0.

Output
For each test case in the input, you should output a line indicating the expected dice throwing times. Output should be rounded to 4 digits after decimal point.

Sample Input
2 0
8 3
2 4
4 5
7 8
0 0

Sample Output
1.1667
2.3441

/*
n个格子,m的特殊格子(可以在不掷骰子的情况下直接从xi飞到yi)，求到终点的期望。
求期望是从后往前推;
*/
#include <algorithm>
#include <iostream>
#include <cmath>
#include <cstdio>
#include <cstring>
#include <map>
#include <vector>
using namespace std;
typedef long long LL;
const LL N = 100000 + 10;
const LL MOD = 1000;
double dp[N];
bool vis[N];
vector<int>ve[N];
int main()
{
    int n, m;
    while(~scanf("%d%d", &n, &m)) {
        if(n + m == 0)
            break;
        int x, y, i, j, tmp;
        memset(dp, 0, sizeof(dp));
        memset(vis, false, sizeof(vis));
        for(i = 0; i <= n; i++)
            ve[i].clear();
        for(i = 0; i < m; i++) {
            scanf("%d%d", &x, &y);
            ve[y].push_back(x);
        }
        for(i = 0; i < ve[n].size(); i++) {
            tmp = ve[n][i];
            dp[tmp] = 0;
            vis[tmp] = true;
        }
        for(i = n - 1; i >= 0; i--) {
            if(!vis[i]) {
                vis[i] = true;
                for(j = i + 1; j <= i + 6; j++)
                    dp[i] += dp[j] / 6;
                dp[i] += 1;
            }
            for(j = 0; j < ve[i].size(); j++) {
                tmp = ve[i][j];
                dp[tmp] = dp[i];
                vis[tmp] = true;
            }
        }
        printf("%.4lf
", dp[0]);
    }
    return 0;
}