HDU1532-Drainage Ditches

Drainage Ditches

Time Limit: 2000/1000 MS (Java/Others) Memory Limit: 65536/32768 K (Java/Others)
Total Submission(s): 18375 Accepted Submission(s): 8677

Problem 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
5 4
1 2 40
1 4 20
2 4 20
2 3 30
3 4 10

Sample Output
50

Source
USACO 93

题目大意：最大流
解题思路： Dinic

#include<cstdio>#include<cstring>#include<queue>#include<cmath>#include<algorithm>using namespace std;const int Ni=205;const int MAXN=1000005;struct Edge{    int u,v,c;    int next;}edge[20*Ni];int n,m;int edn;//边数int p[Ni];//父亲int d[Ni];int sp,tp;//原点，汇点void addedge(int u,int v,int c){    edge[edn].u=u;edge[edn].v=v;edge[edn].c=c;    edge[edn].next=p[u];p[u]=edn++;    edge[edn].u=v;edge[edn].v=u;edge[edn].c=0;    edge[edn].next=p[v];p[v]=edn++;}int bfs()//构造层次网络{    queue<int> q;    memset(d,-1,sizeof(d));//初始化顶点层次为-1    d[sp]=0;    q.push(sp);    while(!q.empty())    {        int cur=q.front();        q.pop();        for(int i=p[cur];i!=-1;i=edge[i].next)        {            int u=edge[i].v;            if(d[u]==-1&&edge[i].c>0)//顶点未被访问过，顶点u，v存在边            {                d[u]=d[cur]+1;//给顶点标记层次                q.push(u);            }        }    }    return d[tp]!=-1;//返回false表明汇点不在层次网络中}int dfs(int a,int b)//进行增广{    int r=0;    if(a==tp) return b;    for(int i=p[a];i!=-1&&r<b;i=edge[i].next)    {        int u=edge[i].v;        if(edge[i].c>0&&d[u]==d[a]+1)        {            int x=min(edge[i].c,b-r);            x=dfs(u,x);            r+=x;            edge[i].c-=x;            edge[i^1].c+=x;        }    }    if(!r) d[a]=-2;    return r;}int dinic(int sp,int tp)//求出最大流{    int total=0,t;    while(bfs())    {        while(t=dfs(sp,MAXN))            total+=t;    }    return total;}int main(){    int i,u,v,c;    while(~scanf("%d%d",&m,&n))    {        edn=0;//初始化        memset(p,-1,sizeof(p));        sp=1;        tp=n;        for(i=0;i<m;i++)        {            scanf("%d%d%d",&u,&v,&c);            addedge(u,v,c);        }        printf("%d\n",dinic(sp,tp));    }    return 0;}