loj#2391 「JOISC 2017 Day 1」港口设施

#include<bits/stdc++.h>
using namespace std;
const int mod = 1e9+7;
vector<int>v[2000100];
int fa[2][2000100],a[2000100],b[2000100],bel[2000100],is[2000100],n,m,vis[2000100];
inline int sf(int wh,int x){return fa[wh][x]==x?x:fa[wh][x]=sf(wh,fa[wh][x]);}
inline void dfs(int x,int f,int d){
    if(vis[x]!=-1){
      if(vis[x]!=d){
          puts("0");
          exit(0);
      }
      return;
    }
    vis[x]=d;
    for(int i=0;i<v[x].size();i++)
      if(v[x][i]!=f)dfs(v[x][i],x,d^1);
}
int main(){
    int i,j,k,Ans=1;
    scanf("%d",&n);
    for(i=1;i<=2*n+1;i++)fa[0][i]=fa[1][i]=i;
    for(i=1;i<=n;i++){
      scanf("%d%d",&a[i],&b[i]);
      bel[a[i]]=bel[b[i]]=i;
      is[b[i]]=1;
      fa[0][b[i]]=b[i]+1;
    }
    for(i=1;i<=2*n;i++)
      if(is[i]){
          int x=a[bel[i]],la=0;
          for(j=sf(0,x+1);j<=i;j=sf(0,j+1)){
            j=sf(1,j);
            v[bel[j]].push_back(bel[i]);
            v[bel[i]].push_back(bel[j]);
            if(la)fa[1][la]=j;
            la=j;
        }
        fa[0][x]=x+1;
      }
    memset(vis,-1,sizeof(vis));
    for(i=1;i<=n;i++)
      if(vis[i]==-1)Ans=Ans*2%mod,dfs(i,0,0);
    cout<<Ans;
    return 0;
}