D38 2-SAT CF27D Ring Road 2

视频链接：D38 2-SAT CF27D Ring Road 2_哔哩哔哩_bilibili

 

 

 

CF27D Ring Road 2 - 洛谷 | 计算机科学教育新生态 (luogu.com.cn)

#include <iostream>
#include <cstring>
#include <algorithm>
using namespace std;const int N=205;
int n,m,x[N],y[N];
int dfn[N],low[N],stk[N],scc[N],tim,top,cnt;
int head[N],idx;
struct Edge{int to,ne;}e[20005];void add(int a,int b){e[++idx].to=b;e[idx].ne=head[a];head[a]=idx;
}
void tarjan(int x){dfn[x]=low[x]=++tim;stk[++top]=x;for(int i=head[x];i;i=e[i].ne){int y=e[i].to;if(!dfn[y]){ //若y尚未访问
      tarjan(y);low[x]=min(low[x],low[y]);}else if(!scc[y]) //若y已访问且未处理low[x]=min(low[x],dfn[y]);}if(low[x]==dfn[x]){ //若x是SCC的根++cnt;for(int y=-1;y!=x;)scc[y=stk[top--]]=cnt;}
}
int main(){scanf("%d%d",&n,&m);for(int i=1;i<=m;i++){scanf("%d%d",&x[i],&y[i]);if(y[i]<x[i]) swap(x[i],y[i]);}for(int i=1;i<=m;i++)for(int j=1;j<=m;j++)if(x[i]<x[j]&&x[j]<y[i]&&y[i]<y[j]||x[j]<x[i]&&x[i]<y[j]&&y[j]<y[i]){add(i,j+m); //i内j外add(i+m,j); //i外j内
      }for(int i=1;i<=2*m;i++) if(!dfn[i]) tarjan(i);for(int i=1;i<=m;i++)if(scc[i]==scc[i+m])return puts("Impossible"),0;for(int i=1;i<=m;i++)scc[i]<scc[i+m]?putchar('i'):putchar('o');
}