POJ 3177 Redundant Paths

边双连通分量+缩点

#include<cstdio>
#include<cstring>
#include<cmath>
#include<vector>
#include<algorithm>
using namespace std;

const int maxn = 10000 + 10; 
const int Maxn = 2 * 100000 + 10; 
int low[maxn];
int dfn[maxn];
int U[maxn], V[maxn];
int flag[maxn];
struct Edge
{
    int from, to, id, ans;
} edge[Maxn];
vector<int>G[maxn];
int N, M;
int tmpdfn;
int tot;
int Start, End;
int TxT[maxn];
int Belong[maxn];
int Block;
int SUM[maxn];

void init()
{
    for (int i = 0; i<maxn; i++) G[i].clear();
    memset(low, 0, sizeof(low));
    memset(dfn, 0, sizeof(dfn));
    memset(flag, 0, sizeof(flag));
    memset(TxT, 0, sizeof(TxT));
    memset(SUM, 0, sizeof SUM);
    memset(Belong, 0, sizeof Belong);
    low[1] = dfn[1] = 1;
    tmpdfn = 0;
    tot = 0;
    Block = 0;
}

void AddEdge(int u, int v)
{
    edge[tot].from = u;
    edge[tot].to = v;
    edge[tot].id = tot;
    edge[tot].ans = 0;
    G[u].push_back(tot);
    tot++;

    edge[tot].from = v;
    edge[tot].to = u;
    edge[tot].id = tot;
    edge[tot].ans = 0;
    G[v].push_back(tot);
    tot++;
}

int Tarjan(int u, int id)
{
    tmpdfn++;
    int lowu = dfn[u] = tmpdfn;
    for (int i = 0; i<G[u].size(); i++)
    {
        int B = G[u][i];
        if (!dfn[edge[B].to])
        {
            int lowv = Tarjan(edge[B].to, edge[B].id);
            lowu = min(lowu, lowv);
            if (lowv >= dfn[u])
            {
                if (lowv>dfn[u])
                    edge[B].ans = 1;
            }
        }
        else if (dfn[edge[B].to])
        {
            if (edge[B].id / 2 == id / 2) continue;
            lowu = min(lowu, dfn[edge[B].to]);
        }
    }
    low[u] = lowu;
    return lowu;
}

void Dfs(int x, int y)
{
    int XZ = 0;
    for (int i = 0; i<G[x].size(); i++)
    {
        int B = G[x][i];
        if (!flag[edge[B].id / 2])
        {
            XZ = 1;
            flag[edge[B].id / 2] = 1;
            TxT[edge[B].to] = 1;
            Belong[edge[B].from] = Block;
            Belong[edge[B].to] = Block;
            Dfs(edge[B].to, y + 1);
        }
    }
    if (!XZ&&!y) Belong[x] = Block;
}

void Slove()
{
    for (int i = 0; i<2 * M; i++)
    if (edge[i].ans)
        flag[edge[i].id / 2] = 1;

    for (int i = Start; i <= End; i++)
    {
        if (!TxT[i])
        {
            TxT[i] = 1;
            Block++;
            Dfs(i, 0);    
        }
    }
}

int main()
{
    while (~scanf("%d%d", &N, &M))
    {
        init();
        for (int i = 0; i < M; i++)
        {
            scanf("%d%d", &U[i], &V[i]);
            AddEdge(U[i], V[i]);
        }
        Start = 1;
        End = N;
        Tarjan(1, -1);
        Slove();
        for (int i = 0; i < M; i++)
            if (Belong[U[i]] != Belong[V[i]])
                SUM[Belong[U[i]]]++, SUM[Belong[V[i]]]++;
        int ANS = 0;
        for (int i = 1; i <= Block; i++) if (SUM[i] == 1) ANS++;
        if (ANS % 2 == 0) printf("%d
", ANS / 2);
        else printf("%d
", ANS / 2 + 1);
    }
    return 0;
}