poj 1273 最大流 ford-fulkerson

#include <iostream>
#include <cstdio>
#include <queue>
#include <cstring>

using namespace std;
#define MAXN 210

struct Matrix
{
  int c,f;     //容量，流量
};

int M,N;
Matrix Edge[MAXN][MAXN];   //流网络
int s,t;

int residual[MAXN][MAXN];   //残留网络
int qu[MAXN*MAXN],qs,qe;   //队列
int pre[MAXN];             //记录增广路的前一个节点
int vis[MAXN];             //标记增广路
int maxflow,min_augment;   //最大流，可改进量

void find_augment_path()   //BFS寻找增广路
{
  int i,cu;
  memset(vis,0,sizeof(vis));
  //  memset(pre,0,sizeof(pre));
  qs = 0;
  qu[qs] = s;
  pre[s] = s;
  vis[s] = 1;
  qe = 1;
  memset(residual,0,sizeof(residual));
  memset(pre,0,sizeof(pre));
  while(qs < qe && pre[t] == 0)
    {
      cu = qu[qs];
      for(int i = 1; i <= N; i++)
	{
	  if(vis[i] == 0)
	    {
	      if(Edge[cu][i].c - Edge[cu][i].f > 0)
		{
		  residual[cu][i] = Edge[cu][i].c - Edge[cu][i].f;
		  pre[i] = cu; qu[qe++] = i; vis[i] = 1;
		}
	      else if(Edge[i][cu].f > 0)
		{
		  residual[cu][i] = Edge[i][cu].f;
		  pre[i] = cu; qu[qe++] = i; vis[i] = 1;
		}
	    }
	}
      qs++;
    }
}

void augment_flow()    //计算改进量
{
  int i = t,j;
  if(pre[i] == 0)  //如果不存在增广路
    {
      min_augment = 0;
      return;
    }
  j = 0x7fffffff;   //设置j为无穷大
  while(i != s)      //倒向追踪
    {
      if(residual[pre[i]][i] < j)   //如果存在残留网络
	{
	  j = residual[pre[i]][i];   //计算增广路上最小残留值即为改进量
	}
      i = pre[i];  
    }
  min_augment = j;
}

void update_flow()    //更新网络流
{
  int i = t;
  if(pre[i] == 0)   //如果没有增广路
    return;
  while(i != s)
    {
      if(Edge[pre[i]][i].c - Edge[pre[i]][i].f > 0)
	{
	  Edge[pre[i]][i].f += min_augment;
	}
      else if(Edge[i][pre[i]].f > 0)
	{
	  Edge[pre[i]][i].f -= min_augment;
	}
      i = pre[i];
    }
}

void Ford()     //增广路算法
{
  s = 1; t = N;
  maxflow = 0;
  while(1)
    {
      find_augment_path();
      augment_flow();
      maxflow += min_augment;
      if(min_augment > 0)    //如果有增广路
	update_flow();
      else              //如果没有增广路
	return;
    }
}

int main()
{
  int i;
  int u,v,c;
  while(scanf("%d%d",&M,&N) != EOF)
    {
      memset(Edge,0,sizeof(Edge));
      for(i = 0; i < M; i++)
	{
	  cin>>u>>v>>c;
	  Edge[u][v].c += c;
	}
      Ford();
      cout<<maxflow<<endl;
    }
  return 0;
}