hdu 6118 最小费用可行流（注意与最大流的区别）

题意：

思路：。。。。注意是可行流，在找增广路的时候条件要进行修改。。。。修改的地方看代码注释

代码：

#include<bits/stdc++.h>
using namespace std;
#define MEM(a,b) memset(a,b,sizeof(a))
#define bug puts("bug");
#define PB push_back
#define MP make_pair
#define X first
#define Y second
typedef unsigned long long ll;
typedef pair<int,int> pii;
const int maxn=4e5+10;
const int mod=1000000007;
using namespace std;
int a[maxn],b[maxn],c[maxn],d[maxn],n,m;

const int MAXN = 10000;
const int MAXM = 100000;
const int INF = 0x3f3f3f3f;
struct Edge{
    int to,next,cap,flow,cost;
} edge[MAXM];
int head[MAXN],tol;
int pre[MAXN],dis[MAXN];
bool vis[MAXN];
int N;
void init(int n){
    N = n;
    tol = 0;
    memset(head,-1,sizeof(head));
}
void addedge(int u,int v,int cap,int cost){
    edge[tol].to = v;
    edge[tol].cap = cap;
    edge[tol].cost = cost;
    edge[tol].flow = 0;
    edge[tol].next = head[u];
    head[u] = tol++;
    edge[tol].to = u;
    edge[tol].cap = 0;
    edge[tol].cost = -cost;
    edge[tol].flow = 0;
    edge[tol].next = head[v];
    head[v] = tol++;
}
bool spfa(int s,int t){
    queue<int>q;
    for(int i = 0; i < N; i++){
        dis[i] = INF;
        vis[i] = false;
        pre[i] = -1;
    }
    dis[s] = 0;
    vis[s] = true;
    q.push(s);
    while(!q.empty()){
        int u = q.front();
        q.pop();
        vis[u] = false;
        for(int i = head[u]; i != -1; i = edge[i].next){
            int v = edge[i].to;
            if(edge[i].cap > edge[i].flow &&dis[v] > dis[u] + edge[i].cost ){
                dis[v] = dis[u] + edge[i].cost;
                pre[v] = i;
                if(!vis[v]){
                    vis[v] = true;
                    q.push(v);
                }
            }
        }
    }
    return dis[t]<=0;//可行流!!!!!!
    //if(pre[t] == -1)return false;
    //else return true;//最大流
}

int minCostMaxflow(int s,int t,int &cost){
    int flow = 0;
    cost = 0;
    while(spfa(s,t)){
        int Min = INF;
        for(int i = pre[t]; i != -1; i = pre[edge[i^1].to]){
            if(Min > edge[i].cap - edge[i].flow)
                Min = edge[i].cap - edge[i].flow;
        }
        for(int i = pre[t]; i != -1; i = pre[edge[i^1].to]){
            edge[i].flow += Min;
            edge[i^1].flow -= Min;
            cost += edge[i].cost * Min;
        }
        flow += Min;
    }
    return flow;
}
int main(){
    while(~scanf("%d%d",&n,&m)){
        init(n+3);
        for(int i=1;i<=n;i++){
            scanf("%d%d%d%d",&a[i],&b[i],&c[i],&d[i]);
            addedge(0,i,b[i],a[i]);
            addedge(i,n+1,d[i],-c[i]);
        }
        int x,y,z;
        for(int i=1;i<=m;i++){
            scanf("%d%d%d",&x,&y,&z);
            addedge(x,y,1e9,z);
            addedge(y,x,1e9,z);
        }
        int ans=0;
        minCostMaxflow(0,n+1,ans);
        cout<<-ans<<endl;
    }
    return 0;
}