网络流 最大流 Dinic 算法

模板：洛谷p3376

#include<bits/stdc++.h>
using namespace std;
typedef long long LL;
const int N=210,M=5e3+10;
int n,m,s,t,d[N],cur[N];
int h[N],e[M<<1],ne[M<<1],id=1;//从2,3开始配对
LL c[M<<1];
void add(int u,int v,LL w){++id;e[id]=v;ne[id]=h[u];c[id]=w;h[u]=id;
}LL dfs(int u,LL mf){//多路增广if(u==t)return mf;LL sum=0;for(int i=cur[u];i;i=ne[i]){cur[u]=i;//当前弧优化int v=e[i];if(d[v]==d[u]+1&&c[i]){LL f=dfs(v,min(mf,c[i]));sum+=f;//累加u的流出流量c[i]-=f;c[i^1]+=f;//更新残留网mf-=f;//减少u的剩余流量if(mf==0)//余量优化break;}}if(sum==0)//残枝优化d[u]=0;return sum;
}bool bfs(){//对点分层，找增广路memset(d,0,sizeof(d));queue<int> q;q.push(s);d[s]=1;while(q.size()){int u=q.front();q.pop();for(int i=h[u];i;i=ne[i]){int v=e[i];if(d[v]==0&&c[i]){d[v]=d[u]+1;q.push(v);if(v==t)return true;}}}return false;
}LL dinic(){//累加可行流LL flow=0;while(bfs()){memcpy(cur,h,sizeof(h));flow+=dfs(s,1e9);}return flow;
}int main(){cin.tie(nullptr)->sync_with_stdio(false);cin>>n>>m>>s>>t;for(int i=1;i<=m;i++){LL u,v,w;cin>>u>>v>>w;add(u,v,w);add(v,u,0);}cout<<dinic()<<endl;return 0;
}