POJ 1966 Cable TV Network【SAP】

http://poj.org/problem?id=1966
POJ 1966 Cable TV Network
算法核心：网络流-SAP

#include<stdio.h>
#include<string.h>
const int N = 50*2+5;
const int INF = N;
const int MAXN=N;
const int maxn = N;
int map[N][N];
int cmap[N][N];
int getNum()
{
 char c;
 int num = 0;
 while(c=getchar())
 {
  if(c<='9'&&c>='0')break;
 }

 while(c<='9'&&c>='0')
 {
  num=num*10+c-'0';
  c = getchar();
 }
 return num;
}

int cur_arc[MAXN],label[MAXN],neck[MAXN];         //当前弧，标号，瓶颈边的入点（姑且这么叫吧）   
int label_count[MAXN],back_up[MAXN],pre[MAXN];    //标号为i节点的数量，cur_flow的纪录,当前流路径中前驱  
//back_up[i]表示当前流流到点i能达到的最大流量
int SAP(int map[][maxn],int v_count,int s,int t)      //邻接矩阵，节点总数，始点，汇点   
{   
 int i;   
 int cur_flow,max_flow,cur;         //当前流，最大流，当前节点，最小标号，临时变量   
 char flag;                                        //标志当前是否有可行流   
 
 memset(label,0,sizeof(label));
 memset(label_count,0,sizeof(label_count));
 memset(cur_arc,0,sizeof(cur_arc));//cur_arc[i]为与i相连最近的点
 label_count[0]=v_count;
 neck[s]=s;//neck[i]表示当前流的瓶颈点
 max_flow = 0;
 cur = s;
 cur_flow = INF;
 //循环代替递归
 while(label[s]<v_count)
 {
    back_up[cur]=cur_flow;
    flag = false;//记录当前点是否存在下一层
    //寻找当前可行流的下一点
    for(i=cur_arc[cur];i<v_count;i++)
    {
     if(map[cur][i]!=0&&label[i]==label[cur]-1)//寻找到下一层
     {
      flag=true;
      cur_arc[cur]=i;//更新
      //更新当前流
      if(map[cur][i]<cur_flow)
      {
       cur_flow = map[cur][i];
       neck[i]=cur;
      }else
       neck[i]=neck[cur];//瓶颈相对前驱节点不变
      pre[i]=cur;
      cur = i;
      if(i==t)//找到可行流
      {
       max_flow+=cur_flow;
       while(cur!=s)
       {
        //当前弧的流量并非无限。。。。若无限，则无需减
        if(map[pre[cur]][cur]!=INF)
        {
         map[pre[cur]][cur]-=cur_flow;
        }
        back_up[cur]-=cur_flow;
        if(map[cur][pre[cur]]!=INF)
         map[cur][pre[cur]]+=cur_flow;
        cur=pre[cur];
       }
       cur = neck[t];//返回至瓶颈节点，继续做
       cur_flow = back_up[cur];
      }
      break;
     }
    }

    if(flag)continue;//下层已经找到
        //未找到下层点
    int nextid;
    int min_label = v_count-1;
    for(i=0;i<v_count;i++)
    {
     if(map[cur][i]!=0&&label[i]<min_label)
     {
      min_label = label[i];
      nextid = i;
     }
    }

    label_count[label[cur]]--;
    if(label_count[label[cur]]==0)break;//GAP优化
    label[cur]=min_label+1;
    cur_arc[cur]=nextid;
    label_count[label[cur]]++;
    if(cur!=s)//从栈中弹出一个节点
    {
     cur = pre[cur];
     cur_flow = back_up[cur];
    }
 }
 return max_flow;
}
int main()
{
   int n,m;
   while(scanf("%d%d",&n,&m)!=EOF)
   {
    memset(map,0,sizeof(map));
    int mid = n;
       while(m--)
    {
     int u =getNum();
     int v = getNum();
     int uu=u+n;
     int vv=v+n;
     map[vv][u]=n+1;
     map[uu][v]=n+1;
    }

    int i,j,t;
    for(i=0;i<n;i++)
     map[i][i+n]=1;

    int ans = n;
    int s = n;
    for(i=0;i<n+n;i++)
     for(j=0;j<n+n;j++)
      cmap[i][j]=map[i][j];
    for(i=1;i<n;i++)
    {
     //if(map[s][i]==0)
     {
      int temp = SAP(cmap,n+n,s,i);
         if(ans>temp)ans = temp;
   for(t=0;t<n+n;t++)
    for(j=0;j<n+n;j++)
     cmap[t][j]=map[t][j];
     }
    }

    printf("%d\n",ans);
   }
   return 0;
}