洛谷 P2740 [USACO4.2]草地排水Drainage Ditches

 P2740 [USACO4.2]草地排水Drainage Ditches

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#include<cstdio>
#include<algorithm>
using namespace std;
#define maxn 10000
#define inf 100000000
int n,m,src,dec,cur[maxn],ans,lev[maxn],front[maxn],head,tail,que[maxn],tot;

struct node{
    int to,next,cap;
}e[maxn];
inline void add(int u,int v,int w)
{
    e[++tot].to=v; e[tot].next=front[u]; e[tot].cap=w; front[u]=tot;
    e[++tot].to=u; e[tot].next=front[v]; e[tot].cap=0; front[v]=tot;
}

inline bool bfs()
{
    for(int i=src;i<=dec;i++) lev[i]=-1,cur[i]=front[i];
    head=tail=0; 
    que[tail++]=src; lev[src]=0;
    while(head<tail)
    {
        for(int i=front[que[head]];i;i=e[i].next)
            if(e[i].cap>0&&lev[e[i].to]==-1)
            {
                lev[e[i].to]=lev[que[head]]+1; 
                que[tail++]=e[i].to;
                if(e[i].to==dec) return true;
            }
        head++;
    }
    return false;
}
inline int dinic(int u,int flow)
{
    if(u==dec) return flow;
    int res=0,delta;
    for(int i=cur[u];i;i=e[i].next)
    {
        if(e[i].cap>0&&lev[e[i].to]>lev[u])
        {
            delta=dinic(e[i].to,min(e[i].cap,flow-res));
            if(delta)
            {
                e[i].cap-=delta; e[i^1].cap+=delta;
                res+=delta; if(res==flow) break; 
            }
        }
    }
    if(res!=flow) lev[u]=-1;
    return res;
}

int main()
{
    scanf("%d%d",&n,&m);
    for(int i=1;i<=n;i++)
    {
        int u,v,w;
        scanf("%d%d%d",&u,&v,&w);
        add(u,v,w);
    }
    src=1; dec=m;
    while(bfs()) 
    ans+=dinic(src,inf);
    printf("%d\n",ans);
    return 0;
}