POJ 1422 Air Raid （二分图最小路径覆盖）

Consider a town where all the streets are one-way and each street leads from one intersection to another. It is also known that starting from an intersection and walking through town’s streets you can never reach the same intersection i.e. the town’s streets form no cycles.

With these assumptions your task is to write a program that finds the minimum number of paratroopers that can descend on the town and visit all the intersections of this town in such a way that more than one paratrooper visits no intersection. Each paratrooper lands at an intersection and can visit other intersections following the town streets. There are no restrictions about the starting intersection for each paratrooper.
Input
Your program should read sets of data. The first line of the input file contains the number of the data sets. Each data set specifies the structure of a town and has the format:

no_of_intersections
no_of_streets
S1 E1
S2 E2
…
Sno_of_streets Eno_of_streets

The first line of each data set contains a positive integer no_of_intersections (greater than 0 and less or equal to 120), which is the number of intersections in the town. The second line contains a positive integer no_of_streets, which is the number of streets in the town. The next no_of_streets lines, one for each street in the town, are randomly ordered and represent the town’s streets. The line corresponding to street k (k <= no_of_streets) consists of two positive integers, separated by one blank: Sk (1 <= Sk <= no_of_intersections) - the number of the intersection that is the start of the street, and Ek (1 <= Ek <= no_of_intersections) - the number of the intersection that is the end of the street. Intersections are represented by integers from 1 to no_of_intersections.

There are no blank lines between consecutive sets of data. Input data are correct.
Output
The result of the program is on standard output. For each input data set the program prints on a single line, starting from the beginning of the line, one integer: the minimum number of paratroopers required to visit all the intersections in the town.
Sample Input
2
4
3
3 4
1 3
2 3
3
3
1 3
1 2
2 3
Sample Output
2
1
**题意：t组数据，对于每组数据，n代表城镇的数目，m代表单向道路数；接下来m行：u v 表示u到v有一条单向路。
现在派伞兵去遍历所有城镇，每个伞兵可以走遍它能走到的任何地方，求最小需要派多少伞兵。
题解：这是个最小路径覆盖问题，在DAG图中寻找尽量少的路径，使得每个节点恰好在一条路径上(不同的路径不可能有公共点)。我们可以发现每匹配一条路径派的伞兵就可以少一个，所以答案等于n-边最大匹配数
**

#include<stdio.h>
#include<string.h>
int n,m,ans;
int g[350][350];
int pipei[350];
int used[350];
int dfs(int x)
{
	for(int i=1;i<=n;i++)
	{
		int y=g[x][i];
		if(y&&!used[i])
		{
			used[i]=1;
			if(pipei[i]==-1||dfs(pipei[i]))
			{
				pipei[i]=x;
				return 1;
			}
		}
	}
	return 0;
}
int solve()
{
	for(int i=1;i<=n;i++)
	{
		memset(used,0,sizeof(used));
		ans+=dfs(i);
	}
}
int main()
{
	int t,u,v;
	scanf("%d",&t);
	while(t--)
	{
		ans=0;
		memset(pipei,-1,sizeof(pipei));
		memset(g,0,sizeof(g));
		scanf("%d %d",&n,&m);
		for(int i=0;i<m;i++)
		{
			scanf("%d %d",&u,&v);
			g[u][v]=1;
		}
		solve();
		printf("%d\n",n-ans);
	}
	
	return 0;
}