Air Raid（二分匹配）

Air Raid

Time Limit : 2000/1000ms (Java/Other) Memory Limit : 65536/32768K (Java/Other)
Total Submission(s) : 11 Accepted Submission(s) : 10
[align=left]Problem Description[/align]
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.

[align=left]Input[/align]
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.

[align=left]Output[/align]
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.

[align=left]Sample Input[/align]

2
4
3
3 4
1 3
2 3
3
3
1 3
1 2
2 3

[align=left]Sample Output[/align]

2
1

[align=left]Source[/align]
Asia 2002, Dhaka (Bengal)

题意： 有N个城市，有M个街道，是单向的，且无回路。

现在要空降一些民来检察完两一个城市。

问至少要多少人。

解法：

这个问题是二分图的最小路径覆盖问题,路径覆盖的定义是:在有向图中找一些路径,使之覆盖了图中的所有顶点,就是任意一个顶点都跟那些路径中的某一条相关联,且任何一个顶点有且只有一条路径与之关联,一个单独的顶点是一条路径……最小路径覆盖就是最少的路径覆盖数.