[USACO4.2]草地排水Drainage Ditches

https://www.luogu.org/problemnew/show/P2740

题解：网络流+最大流

/*
*@Author:   STZG
*@Language: C++
*/
#include <bits/stdc++.h>
#include<iostream>
#include<algorithm>
#include<cstdlib>
#include<cstring>
#include<cstdio>
#include<string>
#include<vector>
#include<bitset>
#include<queue>
#include<deque>
#include<stack>
#include<cmath>
#include<list>
#include<map>
#include<set>
//#define DEBUG
#define RI register int
#define endl "\n"
using namespace std;
typedef long long ll;
//typedef __int128 lll;
const int N=200+10;
const int M=100000+10;
const int MOD=1e9+7;
const double PI = acos(-1.0);
const double EXP = 1E-8;
const int INF = 0x3f3f3f3f;
int s,t,n,m,k,p,l,r,u,v,w,c2;
int ans,cnt,flag,temp,sum;
int dis[N];
struct node{int u,v,c;node(){};node(int form,int to,int cap):u(form),v(to),c(cap){}
};
vector<node>edge;
vector<int> G[N];
void Addedge(int u,int v,int cap){edge.push_back({u,v,cap});edge.push_back({v,u,0});int sz=edge.size();G[u].push_back(sz-2);G[v].push_back(sz-1);
}
bool bfs(int u){memset(dis,-1,sizeof(dis));dis[u]=0;queue<int>q;q.push(u);while(!q.empty()){int u=q.front();q.pop();for(int i=0;i<G[u].size();i++){node e=edge[G[u][i]];//cout<<u<<" "<<e.v<<endl;if(dis[e.v]<0&&e.c>0){dis[e.v]=dis[u]+1;q.push(e.v);}}}return dis[m]>0;
}
int dfs(int u,int flow){if(u==m)return flow;int now;for(int i=0;i<G[u].size();i++){node e=edge[G[u][i]];if(e.c>0&&dis[u]+1==dis[e.v]&&(now=dfs(e.v,min(flow,e.c)))){edge[G[u][i]].c-=now;edge[G[u][i]^1].c+=now;return now;}}return 0;
}
void dinic(){while(bfs(1)){int res=0;while(res=dfs(1,INF)){ans+=res;}}
}
void init(){for(int i=1;i<=m;i++)G[i].clear();edge.clear();ans=0;
}
int main()
{
#ifdef DEBUGfreopen("input.in", "r", stdin);//freopen("output.out", "w", stdout);
#endif//ios::sync_with_stdio(false);//cin.tie(0);//cout.tie(0);//scanf("%d",&t);//int T=0;while(~scanf("%d%d",&n,&m)){init();for(int i=1;i<=n;i++){scanf("%d%d%d",&u,&v,&w);Addedge(u,v,w);}dinic();printf("%d\n",ans);}#ifdef DEBUGprintf("Time cost : %lf s\n",(double)clock()/CLOCKS_PER_SEC);
#endif//cout << "Hello world!" << endl;return 0;
}

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