模板：DINIC

题目见USACO草地排水一题

#include<iostream>#include<cstring>#include<algorithm>#include<cstdio>#include<cmath>#include<queue>#include<vector>#include<climits>#include<string>#include<cstdlib>#include<set>#include<stack>#include<map>#include<bitset>#include<ctime>using namespace std;typedef long long ll;typedef unsigned long long ull;inline ll read(){    char k=0;char ls;ls=getchar();for(;ls<'0'||ls>'9';k=ls,ls=getchar());    ll x=0;for(;ls>='0'&&ls<='9';ls=getchar())x=x*10+ls-'0';    if(k=='-')x=0-x;return x;}ll n,m;struct E{    ll edge;    ll zhi;    ll s;}a[3000];ll last[3000];ll back;ll ans;ll d[3000];inline ll bfs(){    queue<ll>q;    memset(d,0,sizeof(d));    d[1]=1;    q.push(1);    while(!q.empty())    {        ll u=q.front();        q.pop();        for(int j=last[u];j;j=a[j].zhi)        if(a[j].s>0&&d[a[j].edge]==0)        {            d[a[j].edge]=d[u]+1;            q.push(a[j].edge);            if(a[j].edge==n)            return 1;        }    }    return 0;}inline ll dinic(ll x,ll flow){    ll left=flow;    if(x==n)    return flow;    for(int j=last[x];j;j=a[j].zhi)      if(a[j].s>0&&d[a[j].edge]==d[x]+1)        {            ll temp=dinic(a[j].edge,min(left,a[j].s));            if(!temp)d[a[j].edge]=0;            left-=temp;            a[j].s-=temp;            a[j^1].s+=temp;        }    return flow-left;}int main(){    //freopen("dt.in","r",stdin);    //freopen("dt.out","w",stdout);    m=read();n=read();    for(int i=1;i<=m;++i)    {        ll a1=read(),b1=read(),c1=read();         a[++back].edge=b1;//建立正向边         a[back].zhi=last[a1];        a[back].s=c1;        last[a1]=back;        a[++back].edge=a1;//建立逆向边         a[back].zhi=last[b1];        a[back].s=0;        last[b1]=back;    }    while(bfs())    ans+=dinic(1,INT_MAX);    cout<<ans<<endl;    //fclose(stdin);    //fclose(stdout);    return 0;}