bzoj1741 [Usaco2005 nov]Asteroids 穿越小行星群

　　网络流，对于每一个行星，将行星所在行到行星连一条流量为1的边，将行星到其所在列连一条流量为1的边，从源点到所有行连一条流量为1的边，将所有列到汇点都连一条流量为1的边，最大流即为答案。

　　代码

 #include<cstdio>
 #include<algorithm>
 #define N 1000050
 #define M 1000050
 const int inf =0x37373737;
 using namespace std;
 int n,m,i,a,b;
 struct Dinic {
     int s, t, n, pre[N], cur[N], h[N], level[N], sign, q[N];
     int cap[M], to[M], ne[M], flow, e;
     void liu(int u, int v, int w) {
         to[e] = v, ne[e] = h[u], cap[e] = w;
         h[u] = e++;
     }
     void link(int u, int v, int w) {
         liu(u, v, w);
         liu(v, u, );
     }
     void init(int n) {
         for (int i = ; i <= n; ++i)
             h[i] = -;
         e = ;
     }
     bool bfs() {
         int L = , R = ;
         fill(level, level + n, -);
         sign = q[R++] = t;
         level[t] = ;
         while (L < R && level[s] == -) {
             int c = q[L++];
             for (int k = h[c]; ~k; k = ne[k]) {
                 if (cap[k ^ ] >  && level[to[k]] == -)
                     level[to[k]] = level[c] + , q[R++] = to[k];
             }
         }
         return ~level[s];
     }
     void push() {
         int pl = inf, p, k;
         for (p = t; p != s; p = to[k ^ ]) {
             k = pre[p];
             pl = min(pl, cap[k]);
         }
         for (p = t; p != s; p = to[k ^ ]) {
             k = pre[p];
             cap[k] -= pl;
             cap[k ^ ] += pl;
             if (cap[k] == )
                 sign = to[k ^ ];
         }
         flow += pl;
     }
     void dfs(int c) {
         if (c == t)
             push();
         else {
             for (int &k = cur[c]; ~k; k = ne[k])
                 if (cap[k] >  && level[to[k]] +  == level[c]) {
                     pre[to[k]] = k;
                     dfs(to[k]);
                     if (level[sign] > level[c])
                         return;
                     sign = t;
                 }
             level[c] = -;
         }
     }
     int run(int _s, int _t, int _n) {
         s = _s, t = _t, n = _n;
         flow = ;
         while (bfs()) {
             for (int i = ; i < n; ++i)
                 cur[i] = h[i];
             dfs(s);
         }
         return flow;
     }
 } mf;
 int main()
 {
     scanf("%d%d",&n,&m);
     mf.init(*n+m+);
     for (i=;i<=n;i++)
     mf.link(,i,),mf.link(n+i,*n+m+,);
     for (i=;i<=m;i++)
     {
         scanf("%d%d",&a,&b);
         mf.link(a,*n+i,);
         mf.link(*n+i,n+b,);
     }
     printf("%d\n",mf.run(,*n+m+,*n+m+));
 }