hdu 2444 The Accomodation of Students 判断时候构成二分图 + 最大匹配

  1 #include<iostream>
  2 #include<cstdio>
  3 #include<cstring>
  4 #include<algorithm>
  5 #include<queue>
  6 using namespace std;
  7 const int N=205,INF=0x3f3f3f3f;
  8 int bmap[N][N],cx[N],cy[N],dx[N],dy[N];
  9 bool bmask[N];
 10 int flag,vis[N];
 11 int nx,ny,dis,ans;
 12 bool searchpath()
 13 {
 14     queue<int> q;
 15     dis=INF;
 16     memset(dx,-1,sizeof(dx));
 17     memset(dy,-1,sizeof(dy));
 18     for(int i=1;i<=nx;i++)
 19     {
 20         if(cx[i]==-1){ q.push(i); dx[i]=0; }
 21         while(!q.empty())
 22         {
 23             int u=q.front(); q.pop();
 24             if(dx[u]>dis) break;
 25             for(int v=1;v<=ny;v++)
 26             {
 27                 if(bmap[u][v]&&dy[v]==-1)
 28                 {
 29                     dy[v]= dx[u] + 1;
 30                     if(cy[v]==-1) dis=dy[v];
 31                     else
 32                     {
 33                         dx[cy[v]]= dy[v]+1;
 34                         q.push(cy[v]);
 35                     }
 36                 }
 37             }
 38         }
 39     }
 40     return dis!=INF;
 41 }
 42 int findpath(int u)
 43 {
 44     for(int v=1;v<=ny;v++)
 45     {
 46         if(!bmask[v]&&bmap[u][v]&&dy[v]==dx[u]+1)
 47         {
 48             bmask[v]=1;
 49             if(cy[v]!=-1&&dy[v]==dis) continue;
 50             if(cy[v]==-1||findpath(cy[v]))
 51             {
 52                 cy[v]=u; cx[u]=v;
 53                 return 1;
 54             }
 55         }
 56     }
 57     return 0;
 58 }
 59 void maxmatch()
 60 {
 61     ans=0;
 62     memset(cx,-1,sizeof(cx));
 63     memset(cy,-1,sizeof(cy));
 64     while(searchpath())
 65     {
 66         memset(bmask,0,sizeof(bmask));
 67         for(int i=1;i<=nx;i++)
 68             if(cx[i]==-1) ans+=findpath(i);
 69     }
 70 }
 71 void init()
 72 {
 73     memset(bmap,0,sizeof(bmap));
 74 }
 75 void dfs(int u)
 76 {
 77     for(int i=1;i<=nx&&!flag;i++)
 78     {
 79         if(u==i) continue;
 80         if(bmap[u][i]){
 81             if(vis[i]!=-1&&(vis[u]^vis[i]!=1)) flag=1;
 82             if(vis[i]==-1) {vis[i]=vis[u] ^1;dfs(i);}
 83         }
 84     }
 85 }
 86 int main()
 87 {
 88     //freopen("test.txt","r",stdin);
 89     int cas,i,j,k,m,n;
 90     while(scanf("%d%d",&n,&m)!= EOF)
 91     {
 92         init();
 93         memset(vis,-1,sizeof(vis));
 94         flag=0;
 95         for(i=1;i<=m;i++)
 96         {
 97             scanf("%d%d",&j,&k);
 98             bmap[j][k]=1;
 99         }
100         nx=ny=n;
101         vis[1]=0;
102         dfs(1);
103         if(flag) {printf("No\n");continue;}
104         maxmatch();
105         printf("%d\n",ans);
106     }
107     return 0;
108 }