Aizu 2304 Reverse Roads（无向流）

把有向图修改成无向图，并保证每条边的流量守恒并满足有向容量(即abs(flow(u,v) - flow(v,u)) <= 1)满足限制。

得到最大流，根据残流输出答案。

因为最后少了'\n'而WA...

#include<bits/stdc++.h>
using namespace std;
const int N = ,M = N*(N-);
int n,m;
int hd[N],nx[M],to[M],cap[M],ect;

inline void addEdge(int u,int v,int c = )
{
    nx[ect] = hd[u];
    to[ect] = v;
    cap[ect] = c;
    hd[u] = ect++;
}

int S,T;
int vis[N],clk;
int lv[N],q[N];
int cur[N];

bool bfs()
{
    int l = ,r = ;
    clk++;
    q[r++] = S; vis[S] = clk; lv[S] = ;
    while(l<r){
        int u = q[l++];
        for(int i = hd[u]; ~i; i = nx[i]){
            int v = to[i];
            if(vis[v] != clk && cap[i]> ){
                vis[v] = clk;
                lv[v] = lv[u]+;
                q[r++] = v;
            }
        }
    }
    return vis[T] == clk;
}

int aug(int u,int a)
{
    if(u == T||!a) return a;
    int flow = ,f;
    for(int &i = cur[u]; ~i; i = nx[i]){
        int v = to[i];
        if(lv[v] == lv[u]+ && (f = aug(v,min(cap[i],a)))>){
            cap[i] -= f; cap[i^] += f;
            a -= f; flow += f;
            if(!a) break;
        }
    }
    return flow;
}
const int INF = 0x3f3f3f3f;
int MaxFlow()
{
    int flow = ;
    while(bfs()){
        memcpy(cur,hd,sizeof(hd));
        flow += aug(S,INF);
    }
    return flow;
}

//#define LOCAL
int main()
{
#ifdef LOCAL
    freopen("in.txt","r",stdin);
#endif
    scanf("%d%d",&n,&m);
    memset(hd,-,sizeof(hd));
    for(int i = ; i < m; i++){
        int u,v; scanf("%d%d",&u,&v);
        addEdge(v,u);
        addEdge(u,v);
    }
    scanf("%d%d",&S,&T);
    printf("%d\n",MaxFlow());
    vector<int> ans;
    for(int i = ; i < m; i++){
        if(!cap[i<<]) ans.push_back(i+);
    }
    printf("%d\n",(int)ans.size());
    for(auto a:ans){
        printf("%d\n",a);
    }
    return ;
}