POJ 3041 Asteroids

POJ 3041 Asteroids

二分图最大匹配。
以下是我的代码：

#include < iostream >
#include < cstdio >
#include < cstring >
using   namespace  std;

const   int  kMaxn( 50007 );
const   int  kMaxm( 150007 );

struct  Edge
{
     int  u,v;
};

int  nx,ny,m;
int  cnt,first[kMaxn],next[kMaxm];
Edge e[kMaxm];
int  cx[kMaxn],cy[kMaxn];
int  distx[kMaxn],disty[kMaxn];
int  head,tail,q[kMaxn];
int  maxmatch;

void  Clear()
{
    cnt = 0 ;
     for ( int  i = 1 ;i <= nx;i ++ )
        first[i] =- 1 ;
}

void  AddEdge( int  u, int  v)
{
    cnt ++ ;
    e[cnt].u = u;
    e[cnt].v = v;
    next[cnt] = first[u];
    first[u] = cnt;
}

bool  BFS()
{
     bool  re( false );
    head = tail = 0 ;
     for ( int  i = 1 ;i <= nx;i ++ )
        distx[i] = 0 ;
     for ( int  i = 1 ;i <= ny;i ++ )
        disty[i] = 0 ;
     for ( int  i = 1 ;i <= nx;i ++ )
         if (cx[i] ==- 1 )
            q[tail ++ ] = i;
     while (head != tail)
    {
         int  h,t;
         for (h = head,t = tail;h != t;h = (h + 1 ) % kMaxn)
        {
             int  u(q[h]);
             for ( int  i = first[u];i !=- 1 ;i = next[i])
            {
                 int  v(e[i].v);
                 if ( ! disty[v])
                {
                    disty[v] = distx[u] + 1 ;
                     if (cy[v] ==- 1 )
                        re = true ;
                     else
                    {
                        distx[cy[v]] = disty[v] + 1 ;
                        q[tail] = cy[v];
                        tail = (tail + 1 ) % kMaxn;
                    }
                }
            }
        }
        head = t;
    }
     return  re;
}

bool  DFS( int  u)
{
     for ( int  i = first[u];i !=- 1 ;i = next[i])
    {
         int  v(e[i].v);
         if (disty[v] == distx[u] + 1 )
        {
            disty[v] = 0 ;
             if (cy[v] ==- 1   ||  DFS(cy[v]))
            {
                cx[u] = v;
                cy[v] = u;
                 return   true ;
            }
        }
    }
     return   false ;
}

void  HopcroftKarp()
{
    maxmatch = 0 ;
     for ( int  i = 1 ;i <= nx;i ++ )
        cx[i] =- 1 ;
     for ( int  i = 1 ;i <= ny;i ++ )
        cy[i] =- 1 ;
    
     while (BFS())
    {
         for ( int  i = 1 ;i <= nx;i ++ )
             if (cx[i] ==- 1   &&  DFS(i))
                maxmatch ++ ;
    }
}

int  main()
{
     while (scanf( " %d%d " , & nx, & m) == 2 )
    {
        ny = nx;
        Clear();
        
         while (m -- )
        {
             int  u,v;
            scanf( " %d%d " , & u, & v);
            AddEdge(u,v);
        }
        
        HopcroftKarp();
        
        printf( " %d\n " ,maxmatch);
    }
}