POJ2594Treasure Exploration（最小路径覆盖，相交）

Treasure Exploration

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

题解：题意就是让机器人走完所有的路，有向边，问最少需要放几个机器人。
这个很明显是二分图的最小路径覆盖，求一个传递闭包，这个复杂度十分大
至少也是N^3级别了，然后就是节点数-最大匹配数就ok了。

#include<cstdio>
#include<algorithm>
#include<cmath>
#include<cstring>
#include<iostream>
using namespace std;
const int N=507,M=5007;

int n,m;
int cnt,head[N],next[N*N],rea[N*N];
int map[N][N],flag[N],ly[N];

void add(int u,int v){next[++cnt]=head[u],head[u]=cnt,rea[cnt]=v;}
bool dfs(int u)
{
    for (int i=head[u];i!=-1;i=next[i])
    {
        int v=rea[i];
        if (flag[v]) continue;
        flag[v]=true;
        if (!ly[v]||(dfs(ly[v])))
        {
            ly[v]=u;
            return 1;
        }
    }
    return 0;
}
int main()
{
    while(~scanf("%d%d",&n,&m)&&(n|m))
    {
        cnt=0;
        memset(head,-1,sizeof(head));
        memset(map,0,sizeof(map));
        int x,y;
        for (int i=1;i<=m;i++)
        {
            scanf("%d%d",&x,&y);
            map[x][y]=1;
        }
        for (int k=1;k<=n;k++)
            for (int i=1;i<=n;i++)
                for (int j=1;j<=n;j++)
                    if (map[i][k]&&map[k][j]) map[i][j]=1;
        for (int i=1;i<=n;i++)
            for (int j=1;j<=n;j++)
                if (map[i][j]) add(i,j);             
        memset(ly,0,sizeof(ly));
        int ans=0;
        for (int i=1;i<=n;i++)
        {
            memset(flag,0,sizeof(flag));
            ans+=dfs(i);
        }
        ans=n-ans;
        printf("%d
",ans);
    }
}

非要搞个前向星，然后空间还开小了。