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 = 105*2;
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=0;i<maxn;i++)
            G[i].clear();
        edge.clear();
        memset(d,0,sizeof(d));
        memset(vis,0,sizeof(vis));
        memset(cur,0,sizeof(cur));
    }

    void addEdge (int from,int to,int cap)
    {
        edge.push_back((Edge){from,to,cap,0});
        edge.push_back((Edge){to,from,0,0});
        m = edge.size();
        G[from].push_back(m-2);
        G[to].push_back(m-1);
    }

    bool BFS()
    {
        memset(vis,0,sizeof(vis));
        queue<int> Q;
        Q.push(s);
        d[s] = 0;
        vis[s] = 1;
        while(!Q.empty())
        {
            int x = Q.front();
            Q.pop();
            for(int i=0; i<G[x].size(); i++)
            {
                Edge & e = edge[G[x][i]];
                if(!vis[e.to]&&e.cap>e.flow)
                {
                    vis[e.to] = 1;
                    d[e.to] = d[x] + 1;
                    Q.push(e.to);
                }
            }
        }
        return vis[t];
    }

    int DFS(int x,int a)
    {
        if(x==t||a==0) return a;
        int flow = 0,f;
        for(int & i = cur[x]; i<G[x].size(); i++)
        {
            Edge & e = edge[G[x][i]];
            if(d[x] + 1==d[e.to]&&(f=DFS(e.to,min(a,e.cap-e.flow)))>0)
            {
                e.flow +=f;
                edge[G[x][i]^1].flow -=f;
                flow +=f;
                a-=f;
                if(a==0) break;
            }
        }
        return flow;
    }

    int Maxflow (int s,int t) {
        this->s = s;this->t = t;
        int flow = 0;
        while(BFS()) {
            memset(cur,0,sizeof(cur));
            flow+=DFS(s,INF);
        }
        return flow;
    }

}sol;


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

    int s = 0;
    int t = 2*n+1;

    sol.init();
    int sum = 0;
    for(int i=1;i<=n;i++) {
        int a;
        scanf("%d",&a);
        sum +=a;
        sol.addEdge(s,i,a);
        sol.addEdge(i,i+n,INF);
    }

    int sum2 = 0;
    for(int i=1;i<=n;i++) {
        int a;
        scanf("%d",&a);
        sum2+=a;
        sol.addEdge(i+n,t,a);
    }

    for(int i=0;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 0;
}