CodeForces 27D - Ring Road 2 构图2-sat..并输出选择方案
Program:
#include<iostream>
#include<stdio.h>
#include<string.h>
#include<cmath>
#include<queue>
#include<stack>
#include<set>
#include<algorithm>
#define ll long long
#define oo 1000000007
#define pi acos(-1.0)
#define MAXN 205
using namespace std;
struct node
{
int x,y;
}line[MAXN];
vector<int> T[MAXN];
int dfn[MAXN],low[MAXN],DfsIndex,tpnum,tp[MAXN],color[MAXN];
bool instack[MAXN],arc[MAXN][MAXN],d[MAXN];
stack<int> mystack;
bool ok(int a,int b)
{
if (line[a].y>line[b].x && line[a].y<line[b].y)
if (!(line[a].x>=line[b].x && line[a].x<=line[b].y)) return false;
if (line[a].x>line[b].x && line[a].x<line[b].y)
if (!(line[a].y>=line[b].x && line[a].y<=line[b].y)) return false;
return true;
}
void tarjan(int x)
{
int i,y,m=T[x].size();
dfn[x]=low[x]=++DfsIndex;
instack[x]=true;
mystack.push(x);
for (i=0;i<m;i++)
{
y=T[x][i];
if (!dfn[y])
{
tarjan(y);
low[x]=min(low[x],low[y]);
}else
if (instack[y]) low[x]=min(low[x],dfn[y]);
}
if (dfn[x]==low[x])
{
tpnum++;
do
{
x=mystack.top();
mystack.pop();
instack[x]=false;
tp[x]=tpnum;
}while (dfn[x]!=low[x]);
}
return;
}
bool judge(int m)
{
int i;
for (i=0;i<m;i++)
if (tp[i<<1]==tp[(i<<1)|1]) return false;
return true;
}
void dfs(int x,int m)
{
int i;
color[x]=1;
for (i=0;i<(m<<1);i++) if (tp[i]==x) color[tp[i^1]]=-1;
for (i=0;i<tpnum;i++)
if (arc[x][i] && !color[i]) dfs(i,m);
return;
}
int main()
{
int i,j,n,m;
while (~scanf("%d%d",&n,&m))
{
for (i=0;i<m;i++)
{
int x,y,t;
scanf("%d%d",&x,&y);
if (x>y) t=x,x=y,y=t;
line[i].x=x,line[i].y=y;
}
for (i=0;i<(m<<1);i++) T[i].clear();
for (i=0;i<m;i++)
for (j=i+1;j<m;j++)
if (!ok(i,j))
{
T[i<<1].push_back((j<<1)|1);
T[j<<1].push_back((i<<1)|1);
T[(i<<1)|1].push_back(j<<1);
T[(j<<1)|1].push_back(i<<1);
}
memset(instack,false,sizeof(instack));
memset(dfn,0,sizeof(dfn));
while (!mystack.empty()) mystack.pop();
DfsIndex=tpnum=0;
for (i=0;i<(m<<1);i++)
if (!dfn[i]) tarjan(i);
if (!judge(m))
{
printf("Impossible\n");
continue;
}
memset(arc,false,sizeof(arc));
memset(d,0,sizeof(d));
for (i=0;i<(m<<1);i++)
{
int x,num=T[i].size();
for (x=0;x<num;x++)
{
arc[tp[i]][tp[T[i][x]]]=true;
d[tp[T[i][x]]]++;
}
}
memset(color,0,sizeof(color));
for (i=0;i<tpnum;i++)
if (!color[i]) dfs(i,m);
for (i=0;i<m;i++)
if (color[tp[i<<1]]==1) printf("i");
else printf("o");
printf("\n");
}
return 0;
}
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