POJ 3041, Asteroids

Time Limit: 1000MS  Memory Limit: 65536K
Total Submissions: 4468  Accepted: 2319

Description
Bessie wants to navigate her spaceship through a dangerous asteroid field in the shape of an N x N grid (1 <= N <= 500). The grid contains K asteroids (1 <= K <= 10,000), which are conveniently located at the lattice points of the grid.

Fortunately, Bessie has a powerful weapon that can vaporize all the asteroids in any given row or column of the grid with a single shot.This weapon is quite expensive, so she wishes to use it sparingly.Given the location of all the asteroids in the field, find the minimum number of shots Bessie needs to fire to eliminate all of the asteroids.

 

Input
* Line 1: Two integers N and K, separated by a single space.
* Lines 2..K+1: Each line contains two space-separated integers R and C (1 <= R, C <= N) denoting the row and column coordinates of an asteroid, respectively.

 

Output
* Line 1: The integer representing the minimum number of times Bessie must shoot.

 

Sample Input
3 4
1 1
1 3
2 2
3 2

 

Sample Output
2

 

Hint
INPUT DETAILS:
The following diagram represents the data, where "X" is an asteroid and "." is empty space:
X.X
.X.
.X.

OUTPUT DETAILS:
Bessie may fire across row 1 to destroy the asteroids at (1,1) and (1,3), and then she may fire down column 2 to destroy the asteroids at (2,2) and (3,2).

 

Source
USACO 2005 November Gold

---

//  POJ3041.cpp : Defines the entry point for the console application.
//

#include  < iostream >
using   namespace  std;

bool  DFS( int  k,  bool  visited[ 501 ],  int  match[ 501 ],  bool  d[ 501 ][ 501 ],  int  N)
{
     for  ( int  i  =   1 ; i  <=  N;  ++ i)
    {
         if  (visited[i]  ==   false   &&  d[k][i]  ==   true )
        {
            visited[i]  =   true ;
             if  (match[i]  ==   - 1   ||  DFS(match[i],visited,match,d, N))
            {
                match[i]  =  k;
                 return   true ;
            }
        }
    }
     return   false ;
};
int  main( int  argc,  char *  argv[])
{
     int  N, K;
    cin  >>  N  >>  K;
     bool  d[ 501 ][ 501 ];
    memset(d,  0 ,  sizeof (d));
     int  x,y;
     for  ( int  i  =   0 ; i  <  K;  ++ i)
    {
        cin  >>  x  >>  y;
        d[y][x]  =   true ;
    }

     int  match[ 501 ];
    memset(match,  - 1 ,  sizeof (match));
     bool  visited[ 501 ];
     int  cnt  =   0 ;
     for  ( int  i  =   1 ; i  <=  N;  ++ i)
    {
        memset(visited,  0 ,  sizeof (visited));
         if (DFS(i,visited, match, d, N))  ++  cnt;
    }

    cout  <<  cnt  <<  endl;
     return   0 ;
}