51nod 1442 士兵的旅行

拆点，因为只能走一步，那么u->v 后就不能到k了，这样，建图就能保证只走一步；

 #include <bits/stdc++.h>

 using namespace std;

 const int maxn = *;
 const int INF = 0x3f3f3f3f;

 struct Edge {
     int from,to,cap,flow;
 };

 struct Dinic
 {
     int n,m,s,t;
     vector<Edge> edge;
     vector<int> G[maxn];
     bool vis[maxn];
     int d[maxn];
     int cur[maxn];

     void init()
     {
         for(int i=;i<maxn;i++)
             G[i].clear();
         edge.clear();
         memset(d,,sizeof(d));
         memset(vis,,sizeof(vis));
         memset(cur,,sizeof(cur));
     }

     void addEdge (int from,int to,int cap)
     {
         edge.push_back((Edge){from,to,cap,});
         edge.push_back((Edge){to,from,,});
         m = edge.size();
         G[from].push_back(m-);
         G[to].push_back(m-);
     }

     bool BFS()
     {
         memset(vis,,sizeof(vis));
         queue<int> Q;
         Q.push(s);
         d[s] = ;
         vis[s] = ;
         while(!Q.empty())
         {
             int x = Q.front();
             Q.pop();
             for(int i=; i<G[x].size(); i++)
             {
                 Edge & e = edge[G[x][i]];
                 if(!vis[e.to]&&e.cap>e.flow)
                 {
                     vis[e.to] = ;
                     d[e.to] = d[x] + ;
                     Q.push(e.to);
                 }
             }
         }
         return vis[t];
     }

     int DFS(int x,int a)
     {
         if(x==t||a==) return a;
         int flow = ,f;
         for(int & i = cur[x]; i<G[x].size(); i++)
         {
             Edge & e = edge[G[x][i]];
             if(d[x] + ==d[e.to]&&(f=DFS(e.to,min(a,e.cap-e.flow)))>)
             {
                 e.flow +=f;
                 edge[G[x][i]^].flow -=f;
                 flow +=f;
                 a-=f;
                 if(a==) break;
             }
         }
         return flow;
     }

     int Maxflow (int s,int t) {
         this->s = s;this->t = t;
         int flow = ;
         while(BFS()) {
             memset(cur,,sizeof(cur));
             flow+=DFS(s,INF);
         }
         return flow;
     }

 }sol;

 int main()
 {
     int n,m;
     scanf("%d%d",&n,&m);

     int s = ;
     int t = *n+;

     sol.init();
     int sum = ;
     for(int i=;i<=n;i++) {
         int a;
         scanf("%d",&a);
         sum +=a;
         sol.addEdge(s,i,a);
         sol.addEdge(i,i+n,INF);
     }

     int sum2 = ;
     for(int i=;i<=n;i++) {
         int a;
         scanf("%d",&a);
         sum2+=a;
         sol.addEdge(i+n,t,a);
     }

     for(int i=;i<m;i++) {
         int u,v;
         scanf("%d%d",&u,&v);
         sol.addEdge(u,v+n,INF);
         sol.addEdge(v,u+n,INF);
     }

     int ans = sol.Maxflow(s,t);
     if(ans==sum&&sum2==sum)
         puts("YES");
     else puts("NO");

     return ;
 }