UVAL1277_Cops and Thieves

单源点汇点无向图，要阻隔某个点的流量，必须在一个点上消耗一定的价值，问你能否在消耗价值不超过k的前提下，阻隔源点到汇点的流量。

直接对于有权值的点拆点，拆后边容量即为点权。其余的点的容量无穷，最大流即可。

 

 

召唤代码君：

 

 

#include <iostream>
#include <cstring>
#include <cstdio>
#define maxn 5555
#define maxm 555555
using namespace std;

const int inf=11111;
int to[maxm],c[maxm],next[maxm],first[maxn],edge;
int a[maxn],d[maxn],tag[maxn],TAG=222;
bool can[maxn];
int Q[maxm],bot,top;
int n,m,l,s,t;

void _init()
{
    edge=-1;
    for (int i=1; i<=n+n; i++) first[i]=-1;
}

void addedge(int U,int V,int W)
{
    edge++;
    to[edge]=V,c[edge]=W,next[edge]=first[U],first[U]=edge;
    edge++;
    to[edge]=U,c[edge]=0,next[edge]=first[V],first[V]=edge;
}

bool bfs()
{
    Q[bot=top=1]=t,tag[t]=++TAG,d[t]=0,can[t]=false;
    while (bot<=top)
    {
        int cur=Q[bot++];
        for (int i=first[cur]; i!=-1; i=next[i])
            if (c[i^1] && tag[to[i]]!=TAG)
            {
                tag[to[i]]=TAG,d[to[i]]=d[cur]+1;
                can[to[i]]=false,Q[++top]=to[i];
                if (to[i]==s) return true;
            }
    }
    return false;
}

int dfs(int cur,int num)
{
    if (cur==t) return num;
    int tmp=num,k;
    for (int i=first[cur]; i!=-1; i=next[i])
        if (c[i] && d[to[i]]==d[cur]-1 && tag[to[i]]==TAG && !can[to[i]])
        {
            k=dfs(to[i],min(c[i],num));
            if (k) num-=k,c[i]-=k,c[i^1]+=k;
            if (!num) break;
        }
    if (num) can[cur]=true;
    return tmp-num;
}

int main()
{
    int U,V,Flow,K;
    while (scanf("%d",&K)!=EOF)
    {
        scanf("%d%d%d%d",&n,&m,&s,&t);
        for (int i=1; i<=n; i++) scanf("%d",&a[i]);
        _init();
        for (int i=1; i<=m; i++)
        {
            scanf("%d%d",&U,&V);
            addedge(U+n,V,inf),addedge(V+n,U,inf);
        }
        if (s==t)
        {
            puts("NO");
            continue;
        }
        s+=n;
        for (int i=1; i<=n; i++) addedge(i,i+n,a[i]);
        for (Flow=0; Flow<=K && bfs(); bfs()) Flow+=dfs(s,inf);
        if (K>=Flow) puts("YES");
            else puts("NO");
    }
    return 0;
}