POJ1273Drainage Ditches[最大流]

Drainage Ditches

Time Limit: 1000MS		Memory Limit: 10000K
Total Submissions: 71559		Accepted: 27846

Description

Every time it rains on Farmer John's fields, a pond forms over Bessie's favorite clover patch. This means that the clover is covered by water for awhile and takes quite a long time to regrow. Thus, Farmer John has built a set of drainage ditches so that Bessie's clover patch is never covered in water. Instead, the water is drained to a nearby stream. Being an ace engineer, Farmer John has also installed regulators at the beginning of each ditch, so he can control at what rate water flows into that ditch.
Farmer John knows not only how many gallons of water each ditch can transport per minute but also the exact layout of the ditches, which feed out of the pond and into each other and stream in a potentially complex network.
Given all this information, determine the maximum rate at which water can be transported out of the pond and into the stream. For any given ditch, water flows in only one direction, but there might be a way that water can flow in a circle.

Input

The input includes several cases. For each case, the first line contains two space-separated integers, N (0 <= N <= 200) and M (2 <= M <= 200). N is the number of ditches that Farmer John has dug. M is the number of intersections points for those ditches. Intersection 1 is the pond. Intersection point M is the stream. Each of the following N lines contains three integers, Si, Ei, and Ci. Si and Ei (1 <= Si, Ei <= M) designate the intersections between which this ditch flows. Water will flow through this ditch from Si to Ei. Ci (0 <= Ci <= 10,000,000) is the maximum rate at which water will flow through the ditch.

Output

For each case, output a single integer, the maximum rate at which water may emptied from the pond.

Sample Input

Sample Output

Source

USACO 93

最大流裸题

Dinic算法的基本思路:

根据残量网络计算层次图。

在层次图中使用DFS进行增广直到不存在增广路

重复以上步骤直到无法增广

层次图

阻塞流不考虑反向边时的极大流

#include<iostream>
#include<cstdio>
#include<cstring>
#include<algorithm>
#include<cmath>
using namespace std;
const int N=205,M=205,INF=1e9;
int read(){
    char c=getchar();int x=0,f=1;
    while(c<'0'||c>'9'){if(c=='-')f=-1; c=getchar();}
    while(c>='0'&&c<='9'){x=x*10+c-'0'; c=getchar();}
    return x*f;
}
int n,m,u,v,c;
struct edge{
    int v,ne,f,c;
}e[M<<1];
int h[N],cnt=0;
void ins(int u,int v,int c){
    cnt++;
    e[cnt].v=v;e[cnt].c=c;e[cnt].f=0;e[cnt].ne=h[u];h[u]=cnt;
    cnt++;
    e[cnt].v=u;e[cnt].c=0;e[cnt].f=0;e[cnt].ne=h[v];h[v]=cnt;
}
int cur[N],d[N],s,t;
int vis[N],q[N],head=1,tail=1;

int stop=0;
bool bfs(){
    memset(vis,0,sizeof(vis));
    memset(d,0,sizeof(d));
    head=tail=1;
    q[tail++]=s;d[s]=0;vis[s]=1;
    while(head!=tail){
        int u=q[head++];
        for(int i=h[u];i;i=e[i].ne){
            int v=e[i].v;
            if(!vis[v]&&e[i].f<e[i].c){
                q[tail++]=v;vis[v]=1;
                d[v]=d[u]+1;  
                if(v==t) return 1;
            }
        }
    }
    return 0;
}


int dfs(int u,int a){
    if(u==t||a==0) return a;
    int flow=0,f;
    for(int &i=cur[u];i;i=e[i].ne){
        int v=e[i].v;
        if(d[v]==d[u]+1&&(f=dfs(v,min(a,e[i].c-e[i].f)))>0){
            flow+=f;
            e[i].f+=f;
            e[((i-1)^1)+1].f-=f;
            a-=f;
            if(a==0) break;
        }
    }
    return flow;
}
int dinic(){
    int flow=0;
    while(bfs()){
        for(int i=s;i<=t;i++) cur[i]=h[i]; 
        flow+=dfs(s,INF);   
    }
    return flow;
}
int main(){
    while(scanf("%d%d",&m,&n)!=EOF){
        cnt=0;
        memset(h,0,sizeof(h));
        for(int i=1;i<=m;i++){
            u=read();v=read();c=read();
            ins(u,v,c);
        }
        s=1;t=n;
        printf("%d
",dinic());
    }
}