Codeforces Beta Round #27 D. Ring Road 2

题意：有一个环，现给你一些区间的起点与终点，区间可以在环内，可以在环外，但是在环内环外的区间都不能彼此相交，问可能否，如果可能，输出方案。

2-SAT。

#include <iostream>#include <cstdio>#include <cstring>#include <string>#include <vector>#include <algorithm>#include <queue>#include <set>#include <map>using namespace std;typedef long long LL;typedef pair<int,int> PII;const int N=205;const int MaxN=11005;const int MaxE=51005;struct Edge{    int u,v,pre;    Edge(){}    Edge(int u,int v,int pre) :        u(u),v(v),pre(pre) {}}edge[N*10];int hBef[N],hAft[N],nEdge;int n,m,L[N],R[N];bool insta[MaxN];int lab[MaxN],nLab;int sta[MaxN],top;int dfn[MaxN],low[MaxN],idx;int deg[MaxN],color[MaxN],other[MaxN];struct Tarjan_directed{void init(){nLab=top=idx=0;memset(lab,-1,sizeof(lab));memset(dfn,0,sizeof(dfn));memset(low,0,sizeof(low));memset(insta,0,sizeof(insta));memset(deg,0,sizeof(deg));}void dfs(int u){insta[u]=1;sta[top++]=u;dfn[u]=low[u]=++idx;for(int i=hBef[u];i!=-1;i=edge[i].pre){int v=edge[i].v;if(!dfn[v]){dfs(v);low[u]=min(low[u],low[v]);}else if(insta[v]&&dfn[v]<low[u]) low[u]=dfn[v];}if(low[u]==dfn[u]){++nLab; int tmp;do{tmp=sta[--top];insta[tmp]=0;lab[tmp]=nLab;}while(tmp!=u);}}void solve(){    for(int i=0;i<n*2;i++)  if(dfn[i]==0) dfs(i);}}tar;void addEdge(int u,int v,int head[]){    edge[nEdge]=Edge(u,v,head[u]);    head[u]=nEdge++;}void edgeInit(){    nEdge=0;    memset(hBef,-1,sizeof(hBef));    memset(hAft,-1,sizeof(hAft));}bool Intersect(int L1,int R1,int L2,int R2){    if(L1>L2) swap(L1,L2),swap(R1,R2);    if(L1==L2&&R1<R2) swap(L1,L2),swap(R1,R2);    if(R1<=L2) return false;    if(R2<=R1) return false;    return true;}void BuildGraph(){    for(int i=0;i<n;i++)    {        for(int j=i+1;j<n;j++) if(Intersect(L[i],R[i],L[j],R[j]))        {            addEdge(i,j+n,hBef);            addEdge(j,i+n,hBef);            addEdge(i+n,j,hBef);            addEdge(j+n,i,hBef);        }    }}void RebuildGraph(){    int tmp=nEdge;    for(int i=0;i<tmp;i++)    {        int u=lab[edge[i].u],v=lab[edge[i].v];        if(u-v)        {            addEdge(v,u,hAft); deg[u]++;        }    }}bool HasAns(){    for(int i=0;i<n;i++)    {        int u=lab[i],v=lab[i+n];        other[u]=v; other[v]=u;        if(u==v)        {            puts("Impossible");            return false;        }    }    return true;}void TopSort(){    memset(color,0,sizeof(color));    queue<int> que;    for(int i=1;i<=nLab;i++) if(deg[i]==0) que.push(i);    while(que.size())    {        int u=que.front(); que.pop();        if(color[u]==0)        {            color[u]=1;            color[other[u]]=2;        }        for(int i=hAft[u];i!=-1;i=edge[i].pre)        {            int v=edge[i].v;            if(--deg[v]==0) que.push(v);        }    }}void OutputAns(){    for(int i=0;i<n;i++)    {        if(color[lab[i]]==1) putchar('i');        else putchar('o');    }    puts("");}void TwoSAT(){    BuildGraph();    tar.init(); tar.solve();    if(HasAns()==0) return;    RebuildGraph();    TopSort();    OutputAns();}int main(){    while(scanf("%d%d",&m,&n)!=EOF)    {        edgeInit();        for(int i=0;i<n;i++)        {            scanf("%d%d",&L[i],&R[i]);            if(L[i]>R[i]) swap(L[i],R[i]);        }        TwoSAT();    }return 0;}