POJ 3352 Road Construction

Posted 2021-03-10 lsykk

#include<cstdio>
#include<cstring>
#include<cmath>
#include<algorithm>
#define maxn 1005
using namespace std;
int num,to[maxn<<1],nxt[maxn<<1],last[maxn],note[maxn<<1];
int dfn[maxn],low[maxn],cnt;
int col[maxn],tot,cal,ans;
void tarjan(int x,int fa) 
{
    dfn[x]=low[x]=++cnt;
    for (int i=last[x];i;i=nxt[i]) 
    {
        if (!dfn[to[i]]) 
        {
            tarjan(to[i],x);
            low[x]=min(low[x],low[to[i]]);
            if (low[to[i]]>dfn[x]) 
            {
                note[i]=1;
                if (i%2==0) note[i-1]=1;
                else note[i+1]=1;
            }
        }
        else if (dfn[to[i]]<dfn[x] && to[i]!=fa) 
        low[x]=min(low[x],dfn[to[i]]);
    }
}
void dfs(int x)
{
    col[x]=tot;
    for(int i=last[x];i;i=nxt[i])
    {
        if(!col[to[i]] && !note[i]) dfs(to[i]);
        if (note[i]) cal++;
    }
}
void add(int x,int y)
{
    to[++num]=y;nxt[num]=last[x];last[x]=num;note[num]=0;
} 
int main()
{
    int x,y,i,n,m;
    while (scanf("%d%d",&n,&m)!=EOF)
    {
        num=0;cnt=0;tot=0;ans=0;
        memset(last,0,sizeof(last));
        memset(dfn,0,sizeof(dfn));
        memset(col,0,sizeof(col));
        for (i=1;i<=m;i++)
        {
            scanf("%d%d",&x,&y);
            add(x,y);add(y,x);
        }
        for (i=1;i<=n;i++)
            if (!dfn[i]) tarjan(i,0);
        for (i=1;i<=n;i++)
            if (!col[i])
            {
                tot++;cal=0;
                dfs(i);
                if (cal==1) ans++;
            }
        printf("%d
",(ans+1)/2);
    }
    return 0;
}