poj 2594 Treasure Exploration(好题)

Treasure Exploration

Time Limit: 6000MS		Memory Limit: 65536K
Total Submissions: 7408		Accepted: 3029

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
这是个最小路径覆盖问题，但是因为有的点可以重复访问，所以最小路径是可以相交的，我们就用传递闭包建立新图（G’），转化为一般的路径覆盖

#include <map>
#include <set>
#include <stack>
#include <queue>
#include <cmath>
#include <ctime>
#include <vector>
#include <cstdio>
#include <cctype>
#include <cstring>
#include <cstdlib>
#include <iostream>
#include <algorithm>
using namespace std;
#define INF 0x3f3f3f3f
#define inf -0x3f3f3f3f
#define lson l,m,rt<<1
#define rson m+1,r,rt<<1|1
#define mem0(a) memset(a,0,sizeof(a))
#define mem1(a) memset(a,-1,sizeof(a))
#define mem(a, b) memset(a, b, sizeof(a))
typedef long long ll;
const int maxn=501;
int linker[maxn];
int g[maxn][maxn];
int used[maxn];
int uN,vN;

bool dfs(int u){
    for(int v=1;v<=vN;v++){
        if(g[u][v]&&!used[v]){
            used[v]=1;
            if(linker[v]==-1||dfs(linker[v])){
                linker[v]=u;
                return true;
            }
        }
    }
    return false;
}

int hungary(){
    int res=0;
    mem1(linker);
    for(int u=1;u<=uN;u++){
        mem0(used);
        if(dfs(u))
            res++;
    }
    return res;
}

int main(){
    int n,m;
    while(scanf("%d%d",&n,&m)!=EOF){
        if(n==0&&m==0)
            break;
        mem0(g);
        int u,v;
        for(int i=0;i<m;i++){
            scanf("%d%d",&u,&v);
            g[u][v]=1;
        }
        for(int k=1;k<n;k++)
            for(int i=1;i<=n;i++)
                for(int j=1;j<=n;j++){
                    if(g[i][k]+g[k][j]==2)
                        g[i][j]=1;
                }
        uN=vN=n;
        printf("%d\n",n-hungary());
    }
    return 0;
}