Aizu 2304 Reverse Roads 费用流

Reverse Roads

Time Limit: 1 Sec

Memory Limit: 256 MB

题目连接

http://acm.hust.edu.cn/vjudge/contest/view.action?cid=93265#problem/E

Description

ICP city has an express company whose trucks run from the crossing S to the crossing T. The president of the company is feeling upset because all the roads in the city are one-way, and are severely congested. So, he planned to improve the maximum flow (edge disjoint paths) from the crossing S to the crossing T by reversing the traffic direction on some of the roads.

Your task is writing a program to calculate the maximized flow from S to T by reversing some roads, and the list of the reversed roads.

Input

The first line of a data set contains two integers N (2 \leq N \leq 300) and M (0 \leq M \leq {\rm min} (1\,000,\ N(N-1)/2)). N is the number of crossings in the city and M is the number of roads.

The following M lines describe one-way roads in the city. The i-th line (1-based) contains two integers X_i and Y_i (1 \leq X_i, Y_i \leq N,X_i \neq Y_i). X_i is the ID number (1-based) of the starting point of the i-th road and Y_i is that of the terminal point. The last line contains two integers S and T (1 \leq S, T \leq N, S \neq T, 1-based).

The capacity of each road is 1. You can assume that i \neq j implies either X_i \neq X_j or Y_i \neq Y_j, and either X_i \neq Y_j or X_j \neq Y_i.

Output

In the first line, print the maximized flow by reversing some roads. In the second line, print the number R of the reversed roads. In each of the following R lines, print the ID number (1-based) of a reversed road. You may not print the same ID number more than once.

If there are multiple answers which would give us the same flow capacity, you can print any of them.

Sample Input

Sample Output

1

0

HINT

题意

给你一个图，然后图的边可以反转，反转的代价为1，问你保证最大流的情况下，使得反转的边最少

并且把反转的边输出出来

题解：

裸的费用流，特别裸……

代码：

//qscqesze

#include <cstdio>

#include <cmath>

#include <cstring>

#include <ctime>

#include <iostream>

#include <algorithm>

#include <set>

#include <bitset>

#include <vector>

#include <sstream>

#include <queue>

#include <typeinfo>

#include <fstream>

#include <map>

#include <stack>

typedef long long ll;

using namespace std;

//freopen("D.in","r",stdin);

//freopen("D.out","w",stdout);

#define sspeed ios_base::sync_with_stdio(0);cin.tie(0)

#define maxn 200500

#define mod 1001

#define eps 1e-9

#define pi 3.1415926

int Num;

//const int inf=0x7fffffff;

const ll inf=;

inline ll read()

{

    ll x=,f=;char ch=getchar();

    while(ch<''||ch>''){if(ch=='-')f=-;ch=getchar();}

    while(ch>=''&&ch<=''){x=x*+ch-'';ch=getchar();}

    return x*f;

}

//*************************************************************************************

const int MAXN = ;

const int MAXM = ;

const int INF = 0x3f3f3f3f;

struct Edge

{

    int to, next, cap, flow, cost, id;

    int x, y;

} edge[MAXM],HH[MAXN],MM[MAXN];

int head[MAXN],tol;

int pre[MAXN],dis[MAXN];

bool vis[MAXN];

int N, M;

char map[MAXN][MAXN];

void init()

{

    N = MAXN;

    tol = ;

    memset(head, -, sizeof(head));

}

void addedge(int u, int v, int cap, int cost,int id)//左端点，右端点，容量，花费, 编号

{

    edge[tol]. to = v;

    edge[tol]. cap = cap;

    edge[tol]. cost = cost;

    edge[tol]. flow = ;

    edge[tol]. next = head[u];

    edge[tol].id = id;

    head[u] = tol++;

    edge[tol]. to = u;

    edge[tol]. cap = ;

    edge[tol]. cost = -cost;

    edge[tol]. flow = ;

    edge[tol]. next = head[v];

    edge[tol].id = id;

    head[v] = tol++;

}

bool spfa(int s, int t)

{

    queue<int>q;

    for(int i = ; i < N; i++)

    {

        dis[i] = INF;

        vis[i] = false;

        pre[i] = -;

    }

    dis[s] = ;

    vis[s] = true;

    q.push(s);

    while(!q.empty())

    {

        int u = q.front();

        q.pop();

        vis[u] = false;

        for(int i = head[u]; i != -; i = edge[i]. next)

        {

            int v = edge[i]. to;

            if(edge[i]. cap > edge[i]. flow &&

                    dis[v] > dis[u] + edge[i]. cost )

            {

                dis[v] = dis[u] + edge[i]. cost;

                pre[v] = i;

                if(!vis[v])

                {

                    vis[v] = true;

                    q.push(v);

                }

            }

        }

    }

    if(pre[t] == -) return false;

    else return true;

}

//返回的是最大流， cost存的是最小费用

vector<int> ans;

int minCostMaxflow(int s, int t, int &cost)

{

    int flow = ;

    cost = ;

    while(spfa(s,t))

    {

        int Min = INF;

        for(int i = pre[t]; i != -; i = pre[edge[i^]. to])

        {

            if(Min > edge[i]. cap - edge[i]. flow)

                Min = edge[i]. cap - edge[i]. flow;

        }

        for(int i = pre[t]; i != -; i = pre[edge[i^]. to])

        {

            edge[i]. flow += Min;

            edge[i^]. flow -= Min;

            cost += edge[i]. cost * Min;

        }

        flow += Min;

    }

    return flow;

}

vector<int> Q;

int main()

{

    init();

    int n=read(),m=read();

    for(int i=;i<m;i++)

    {

        int x=read(),y=read();

        addedge(x,y,,,i+);

        addedge(y,x,,,i+);

    }

    int s=read(),t=read();

    int ans1 = ,ans2 = ;

    ans1 = minCostMaxflow(s,t,ans2);

    for(int i=;i<=n;i++)

    {

        for(int j = head[i]; j != -; j = edge[j]. next)

        {

            if(edge[j].flow == )

            {

                if(edge[j].cost)

                {

                    Q.push_back(edge[j].id);

                }

            }

        }

    }

    printf("%d\n%d\n",ans1,ans2);

    for(int i=;i<Q.size();i++)

        printf("%d\n",Q[i]);

}