Drainage Ditches USACO 4.2 (最大流，BFS)

算法导论求最大流这一节还没看过，下午看了下，参照了大牛的代码模仿写了Ford-Fulkerson+BFS过了...

/*

ID: hubiao cave

PROG: ditch

LANG: C++

*/




#include<iostream>

#include<fstream>

#include<cstring>
#include<queue>
#include<deque>

using namespace std;



int E[201][201];
int m,n;
bool visited[201];
int prenumber;
queue<pair<int,int> > intqueue;
deque<pair<int,int> >intde;

void BFS();
void PreBFS();
int main()

{


    ifstream fin("ditch.in");

    ofstream fout("ditch.out");

    fin>>n>>m;
    for(int i=1;i<=n;i++)
    {
        int x,y,z;
        fin>>x>>y>>z;
        E[x][y]+=z;
    }
    int ans=0;
    for(;;)
    {
        PreBFS();
        BFS();
        if(prenumber==0)
            break;
        int min=2111111111;
        
        int pre=intde.back().second;
        do
        {
            while(intde.back().second!=pre)
                intde.pop_back();
            pair<int,int>cur=intde.back();
            intde.pop_back();
            if(E[cur.first][cur.second]<min&&cur.first!=0)
                min=E[cur.first][cur.second];
            if(cur.first!=0)
            intde.push_front(cur);
            pre=cur.first;
        }while(pre!=0);
        
        ans+=min;
        while(!intde.empty())
        {
            pair<int,int> pii=intde.front();
            intde.pop_front();
            E[pii.first][pii.second]-=min;
            E[pii.second][pii.first]+=min;
        }
    }

    fout<<ans<<endl;
    return 0;


}

void BFS()
{
    intqueue.push(make_pair(0,1));
    while(!intqueue.empty())
    {
        pair<int,int> cur=intqueue.front();    
        if(visited[cur.second])
        {
            intqueue.pop();
            continue;
        }
        if(cur.second==m)
        {
            prenumber=cur.first;
            intde.push_back(cur);
            return;
        }
        visited[cur.second]=true;
        intde.push_back(cur);
        intqueue.pop();
        for(int i=1;i<=m;i++)
        {
            if(E[cur.second][i]&&!visited[i])
            {
                intqueue.push(make_pair(cur.second,i));
            }
        }
    }
}

void PreBFS()
{
    while(!intqueue.empty())
    {
        intqueue.pop();
    }
    intde.clear();
    prenumber=0;
    memset(visited,0,sizeof(visited));

}