POJ 2594

Treasure Exploration

Time Limit: 6000MS		Memory Limit: 65536K
Total Submissions: 6195		Accepted: 2500

Description

Have you ever read any book about treasure exploration? Have you ever see any film about treasure exploration? Have you ever explored treasure? If you never have such experiences, you would never know what fun treasure exploring brings to you. 
Recently, a company named EUC (Exploring the Unknown Company) plan to explore an unknown place on Mars, which is considered full of treasure. For fast development of technology and bad environment for human beings, EUC sends some robots to explore the treasure. 
To make it easy, we use a graph, which is formed by N points (these N points are numbered from 1 to N), to represent the places to be explored. And some points are connected by one-way road, which means that, through the road, a robot can only move from one end to the other end, but cannot move back. For some unknown reasons, there is no circle in this graph. The robots can be sent to any point from Earth by rockets. After landing, the robot can visit some points through the roads, and it can choose some points, which are on its roads, to explore. You should notice that the roads of two different robots may contain some same point. 
For financial reason, EUC wants to use minimal number of robots to explore all the points on Mars. 
As an ICPCer, who has excellent programming skill, can your help EUC?

Input

The input will consist of several test cases. For each test case, two integers N (1 <= N <= 500) and M (0 <= M <= 5000) are given in the first line, indicating the number of points and the number of one-way roads in the graph respectively. Each of the following M lines contains two different integers A and B, indicating there is a one-way from A to B (0 < A, B <= N). The input is terminated by a single line with two zeros.

Output

For each test of the input, print a line containing the least robots needed.

Sample Input

1 0
2 1
1 2
2 0
0 0

Sample Output

1
1
2

Source

POJ Monthly--2005.08.28,Li Haoyuan

一个星球上有很多点，星球是一个DGA，现在要派出尽量少的人能把所有点都走一遍，一开始以为是最小路径覆盖，但是题目说一个点上可以同时有多个人，而最小路径覆盖是用不相交的路径覆盖一个图。所以我们要先对图求一次传递闭包，再求最小路径覆盖。

#include<cstdio>
#include<cstring>
#define maxn 509
using namespace std;
int n,m;
int a[maxn][maxn],cx[maxn],cy[maxn],vis[maxn];
int path(int u)
{
    int v;
    for(v=1;v<=n;v++)
    {
        if(a[u][v]&&!vis[v])
        {
            vis[v]=1;
            if(cy[v]==-1||path(cy[v]))
            {
                cx[u]=v;
                cy[v]=u;
                return 1;
            }
        }
    }
    return 0;
}
int match()
{
    int res=0;
    memset(cx,-1,sizeof(cx));
    memset(cy,-1,sizeof(cy));
    for(int i=1;i<=n;i++)
    {
        if(cx[i]==-1)
        {
            memset(vis,0,sizeof(vis));
            res+=path(i);
        }
    }
    return res;
}
int main()
{
    while(scanf("%d%d",&n,&m),n+m)
    {
        memset(a,0,sizeof(a));
        for(int i=0;i<m;i++)
        {
            int x,y;
            scanf("%d%d",&x,&y);
            a[x][y]=1;
        }
        for(int k=1;k<=n;k++)
            for(int i=1;i<=n;i++)
                for(int j=1;j<=n;j++)
            a[i][j]|=a[i][k]&a[k][j];
        printf("%d\n",n-match());
    }
}