HDU 1532 Drainage Ditches（最大流）

Drainage Ditches

Time Limit:1000MS     Memory Limit:10000KB     64bit IO Format:%I64d & %I64u

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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

题意：有n条边单向边，m个点，1为起点，m为终点。告诉每两点之间的流量，求起点到终点的最大流量。

第一次接触网络流。虽然这是一道裸的网络流求最大流，直接上的大白书的模板。但是因为忘了在主函数里面赋初值，错了很多发。。。

代码：

#include<queue>
#include<vector>
#include<stdio.h>
#include<string.h>
#include<algorithm>
#define maxn 205
#define INF 0x7f7f7f7f
using namespace std;
typedef long long LL;
struct Edge
{
    LL from,to,cap,flow;
};
LL n,m,s,t;
vector<Edge> edges;
vector<LL> G[maxn];
bool vis[maxn];
LL d[maxn];
LL cur[maxn];

bool BFS()
{
    memset(vis,0,sizeof(vis));
    queue<LL> Q;
    Q.push(s);
    d[s]=0;
    vis[s]=1;
    while(!Q.empty())
    {
        LL x=Q.front();
        Q.pop();
        for(LL i=0;i<G[x].size();i++)
        {
            Edge&e=edges[G[x][i]];
            if(!vis[e.to]&&e.cap>e.flow)
            {
                vis[e.to]=1;
                d[e.to]=d[x]+1;
                Q.push(e.to);
            }
        }
    }
    return vis[t];
}

void AddEdge(LL from,LL to,LL cap)
{
    edges.push_back((Edge){from,to,cap,0});
    edges.push_back((Edge){to,from,0,0});
    int len=edges.size();
    G[from].push_back(len-2);
    G[to].push_back(len-1);
}
LL DFS(LL x,LL a)
{
    if(x==t||a==0)
        return a;
    LL flow=0,f;
    for(LL &i=cur[x];i<G[x].size();i++)
    {
        Edge&e=edges[G[x][i]];
        if(d[x]+1==d[e.to]&&(f=DFS(e.to,min(a,e.cap-e.flow)))>0)
        {
            e.flow+=f;
            edges[G[x][i]^1].flow-=f;
            flow+=f;
            a-=f;
            if(a==0)
                break;
        }
    }
    return flow;
}
LL Maxflow()
{
    LL flow=0;
    while(BFS())
    {
        memset(cur,0,sizeof(cur));
        flow+=DFS(s,INF);
    }
    return flow;
}
int main()
{
    while(~scanf("%lld%lld",&n,&m))
    {
        s=1;
        t=m;
        edges.clear();
        for(int i=1;i<=n;i++)
            G[i].clear();
        for(LL i=0;i<n;i++)
        {
            LL st,ed,cap;
            scanf("%lld%lld%lld",&st,&ed,&cap);
            AddEdge(st,ed,cap);
        }
        printf("%lld\n",Maxflow());
    }
    return 0;
}