poj 2186 Popular Cows 有向图强连通分量 tarjan

来源：互联网发布：淘宝行业数据哪里看编辑：程序博客网时间：2024/06/05 09:01
#include <stdio.h>#include <string.h>#include <iostream>#include <algorithm>#include <vector>#include <queue>#include <stack>#include <set>#include <map>#include <string>#include <math.h>#include <stdlib.h>#include <time.h>//#include<bits/stdc++.h>using namespace std;typedef long long ll;const int N=100000+5;struct Edge{    int s,t;};vector<Edge>edge;stack<int>st;vector<int>g[100000+5];int n,m,in[N],id[N],dfn[N],low[N],def,cnt;void Addedge(int u,int v){    Edge tp;    tp.s=u,tp.t=v;    edge.push_back(tp);    g[u].push_back(edge.size()-1);}void dfs(int u){    int k,mx=dfn[u]=low[u]=++def;    st.push(u);    for(int i=0; i<g[u].size(); i++)    {        int v=edge[g[u][i]].t;        if(dfn[v]==-1) dfs(v);        if(low[v]<mx) mx=low[v];    }    if(mx<low[u])    {        low[u]=mx;        return ;    }    do    {        k=st.top();        id[k]=cnt;        low[k]=n;        st.pop();    }    while(k!=u);    cnt++;}int main(){    int i,j,u,v,ans,pos;    //freopen("in.txt","r",stdin);    while(~scanf("%d%d",&n,&m))    {        for(i=0; i<n; i++)        {            g[i].clear();            id[i]=i;            in[i]=0;        }        edge.clear();        for(i=0; i<m; i++)        {            scanf("%d%d",&u,&v);            Addedge(u-1,v-1);        }        while(!st.empty()) st.pop();        def=0;        cnt=0;        memset(dfn,0xff,sizeof(dfn));        for(i=0; i<n; i++)        {            if(dfn[i]==-1)                dfs(i);        }        for(i=0; i<m; i++)        {            u=id[edge[i].s];            v=id[edge[i].t];            if(u!=v) in[u]++;            //printf("%d %d\n",u,v);        }        ans=0;        pos=-1;        for(i=0; i<cnt; i++)        {            if(in[i]==0)            {                //printf("%d\n",id[i]);                if(pos==-1)                {                    ans++;                    pos=i;                }                else if(pos!=i) ans++;            }        }        // printf("%d\n",pos);        if(ans!=1) printf("0\n");        else        {            ans=0;            for(i=0; i<n; i++)            {                if(id[i]==pos)                    ans++;            }            printf("%d\n",ans);        }    }    return 0;}
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