Drainage Ditches HDU - 1532

问题：

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

这是一个最大流模板题，需要注意的是存在重边，需要把重边的水流速度叠加。

代码：

#define N 210
#include<queue>
#include<stdio.h>
#include<string.h>
#include<algorithm>
using namespace std;
int k[N][N],p[N],book[N];
int s,t,n,m,maxflow,minaug;
struct node
{
    int c,f;
}e[N][N];
void find()//求增广路
{
    queue<int>q;
    memset(book,0,sizeof(book));
    memset(p,0,sizeof(p));
    memset(k,0,sizeof(k));
    p[s]=s;book[s]=1;
    q.push(s);
    while(!q.empty()&&p[t]==0)
    {
        int cu=q.front();
        q.pop();
        for(int i=1; i<=n; i++)//建立残留网络
        {
            if(!book[i])
            {
                if(e[cu][i].c-e[cu][i].f>0)//总流量-已用流量
                {
                    k[cu][i]=e[cu][i].c-e[cu][i].f;
                    q.push(i);p[i]=cu;book[i]=1;
                }
                else if(e[i][cu].f>0)
                {
                    k[cu][i]=e[i][cu].f;
                    q.push(i);p[i]=cu;book[i]=1;
                }
            }
        }
    }
}
void augflow()
{
    int i=t,j;
    if(p[i]==0)
    {
        minaug=0;return ;
    }
    j=0x7fffffff;
    while(i!=s)
    {
        if(k[p[i]][i]<j)
            j=k[p[i]][i];//计算增广路改进的最小流量
        i=p[i];
    }
    minaug=j;
}
void upflow()//调整流量，改变已用流量的网络图
{
    int i=t;
    if(p[t]==0)return;
    while(i!=s)
    {
        if(e[p[i]][i].c-e[p[i]][i].f>0)
            e[p[i]][i].f+=minaug;
        else if(e[p[i]][i].f>0)
            e[p[i]][i].f+=minaug;
        i=p[i];
    }
}
void slove()
{
    s=1; t=n;
    maxflow=0;
    while(1)
    {
        find();
        augflow();//计算可改进量
        maxflow+=minaug;
        if(minaug>0)upflow();
        else return;
    }
}
int main()
{
    int a,b,c;
    while(~scanf("%d%d",&m,&n))
    {
        memset(e,0,sizeof(e));
        for(int i=0; i<m; i++)
        {
            scanf("%d%d%d",&a,&b,&c);
            e[a][b].c+=c;
        }
        slove();
        printf("%d\n",maxflow);
    }
    return 0;
}