UVA 820 Internet Bandwidth 因特网宽带（无向图，最大流，常规）

题意：给一个无向图，每条边上都有容量的限制，要求求出给定起点和终点的最大流。

思路：每条无向边就得拆成2条，每条还得有反向边，所以共4条。源点汇点已经给出，所以不用建了。直接在图上跑最大流就可以了。

#include <bits/stdc++.h>
#define LL long long
#define pii pair<int,int>
#define INF 0x7f7f7f7f
using namespace std;
const int N=200;
const int mod=1e9+7;
int s, t;

int path[N], flow[N];
vector<int> vect[N];

struct node
{
    int from, to, cap, flow;
    node(){};
    node(int from,int to,int cap,int flow):from(from),to(to),cap(cap),flow(flow){};
}edge[100000];
int edge_cnt;

void add_node(int from,int to,int cap,int flow)
{
    edge[edge_cnt]=node(from,to,cap,flow);
    vect[from].push_back(edge_cnt++);
}

int BFS(int s,int e)
{
    deque<int> que(1,s);
    flow[s]=INF;
    while(!que.empty())
    {
        int x=que.front();
        que.pop_front();
        for(int i=0; i<vect[x].size(); i++)
        {
            node e=edge[vect[x][i]];
            if(!flow[e.to] && e.cap>e.flow)
            {
                flow[e.to]=min(flow[e.from],e.cap-e.flow);
                path[e.to]=vect[x][i];
                que.push_back(e.to);
            }
        }
        if(flow[e]) return flow[e];
    }
    return flow[e];
}

int max_flow(int s,int e)
{
    int ans_flow=0;
    while(true)
    {
        memset(path,0,sizeof(path));
        memset(flow,0,sizeof(flow));

        int tmp=BFS(s,e);
        if(!tmp)    return ans_flow;
        ans_flow+=tmp;

        int ed=e;
        while(ed!=s)
        {
            int t=path[ed];
            edge[t].flow+=tmp;
            edge[t^1].flow-=tmp;
            ed=edge[t].from;
        }
    }
}
int main()
{
    freopen("input.txt", "r", stdin);
    int n, a, b, v, c, j=0;
    while(scanf("%d",&n),n)
    {
        edge_cnt=0;
        memset(edge,0,sizeof(edge));
        for(int i=0; i<=n+1; i++)   vect[i].clear();

        scanf("%d%d%d", &s, &t, &c);
        for(int i=0; i<c; i++)
        {
            scanf("%d%d%d",&a,&b,&v);
            add_node(a, b, v, 0);
            add_node(b, a, 0, 0);
            add_node(b, a, v, 0);
            add_node(a, b, 0, 0);
        }
        printf("Network %d
",++j);
        printf("The bandwidth is %d.

", max_flow(s ,t) );
    }
    return 0;
}