poj 1273 Drainage Ditches

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题目链接：

http://poj.org/problem?id=1273

题解：

网络流模板

代码：

EK算法：

#include <cmath>#include <vector>#include <queue>#include <cstdio>#include <cstring>#include <iostream>#include <algorithm>using namespace std;#define inf 0x3f3f3f3f#define met(a,b) memset(a,b,sizeof(a))const int maxn = 200+10;int mp[maxn][maxn];bool visited[maxn];int pre[maxn];int n,m;int bfs(int s,int t){    met(visited,0);    met(pre,0);    pre[s]=s;    visited[s]=true;    queue<int>q;    while(!q.empty())        q.pop();    q.push(s);    while(!q.empty())    {        int p=q.front();        q.pop();        for(int i=1;i<=m;i++)        {            if(mp[p][i]&&!visited[i])            {                visited[i]=1;                pre[i]=p;                if(i==t)                    return true;                q.push(i);            }        }    }    return false;}int EX(int s,int t){    int ans=0,flow;    while(bfs(s,t))    {        flow=inf;        for(int i=t;i!=s;i=pre[i])            flow=min(flow,mp[pre[i]][i]);        for(int i=t;i!=s;i=pre[i])        {            mp[pre[i]][i]-=flow;            mp[i][pre[i]]+=flow;        }        ans+=flow;    }    return ans;}int main(){    while(scanf("%d%d",&n,&m)!=EOF)    {        met(mp,0);        for(int i=0;i<n;i++)        {            int x,y,z;            scanf("%d%d%d",&x,&y,&z);            mp[x][y]+=z;        }        printf("%d\n",EX(1,m));    }}

Dinic算法：

#include <cmath>#include <vector>#include <queue>#include <cstdio>#include <cstring>#include <iostream>#include <algorithm>using namespace std;#define inf 0x7fffffff#define met(a,b) memset(a,b,sizeof(a))const int maxn = 200+100;int mp[maxn][maxn];int dis[maxn];int s,t;int n,m;int bfs(){    met(dis,0);    queue<int>q;    while(!q.empty())        q.pop();    dis[s]=1;    q.push(s);    while(!q.empty())    {        int p=q.front();        q.pop();        for(int i=1;i<=m;i++)        {            if(mp[p][i]&&!dis[i])            {                dis[i]=dis[p]+1;                q.push(i);            }        }    }    return dis[t]==0;}int dfs(int pos,int flow){    if(pos==t)        return flow;    int temp=flow;    for(int i=1;i<=m&&temp;i++)    {        if(mp[pos][i]&&(dis[i]==dis[pos]+1))        {            int a=dfs(i,min(mp[pos][i],temp));            mp[pos][i]-=a;            mp[i][pos]+=a;            temp-=a;        }    }    return flow-temp;}int Dinic(){    int ans=0,temp=0;    while(!bfs())    {        while(temp=dfs(1,inf))            ans+=temp;    }    return ans;}int main(){    while(scanf("%d%d",&n,&m)!=EOF)    {        met(mp,0);        for(int i=0;i<n;i++)        {            int x,y,z;            scanf("%d%d%d",&x,&y,&z);            mp[x][y]+=z;        }        s=1,t=m;        printf("%d\n",Dinic());    }}

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