HDU 3622 Bomb Game(二分+2SAT)

#include <cstdio>
#include <cmath>
#include <vector>
#include <cstring>
using namespace std;
const int maxn =105;
struct TwoSAT
{
    int n;
    vector<int> G[maxn*2];
    bool mark[maxn*2];
    int S[maxn*2],c;
    bool dfs(int x)
    {
        if(mark[x^1]) return false;
        if(mark[x]) return true;
        mark[x]=true;
        S[c++]=x;
        for(int i=0; i<G[x].size(); ++i)
            if(!dfs(G[x][i])) return false;
        return true;
    }
    void init(int n)
    {
        this->n=n;
        for(int i=0; i<n*2; ++i) G[i].clear();
        memset(mark,0,sizeof(mark));
    }
    void add_clause(int x,int xval,int y,int yval)
    {
        x=x*2+xval;
        y=y*2+yval;
        G[x^1].push_back(y);
        G[y^1].push_back(x);
    }
    bool solve()
    {
        for(int i=0; i<n*2; i+=2)
        {
            if(!mark[i]&&!mark[i+1])
            {
                c=0;
                if(!dfs(i))
                {
                    while(c>0) mark[S[--c]]=false;
                    if(!dfs(i+1)) return false;
                }
            }
        }
        return true;
    }
};
struct Point
{
    int x,y;
};
Point p[105][2];
int n;
TwoSAT solver;
double distan(Point a,Point b)
{
    return sqrt((a.x*1.0-b.x)*(a.x-b.x)+(a.y-b.y)*(a.y-b.y));
}
bool judge(double d)
{
    solver.init(n);
    for(int i=0; i<n; ++i)for(int a=0; a<2; ++a)
            for(int j=i+1; j<n; ++j)for(int b=0; b<2; ++b)
                    if(distan(p[i][a],p[j][b])<=2*d)
                        solver.add_clause(i,a^1,j,b^1);
    return solver.solve();
}
int main()
{
    while(scanf("%d",&n)!=EOF)
    {

        for(int i=0; i<n; ++i)
        {
            for(int j=0; j<2; ++j)
                scanf("%d%d",&p[i][j].x,&p[i][j].y);
        }
        double maxi=0;
        solver.init(n);
        for(int i=0; i<n; ++i)for(int a=0; a<2; ++a)
                for(int j=i+1; j<n; ++j)for(int b=0; b<2; ++b)
                        maxi=max(maxi,distan(p[i][a],p[j][b]));
        double L=0,R=maxi;
        for(int i=0; i<30; ++i)
        {
            double mid=(L+R)/2;
            if(judge(mid)) L=mid;
            else R=mid;
        }
        printf("%.2lf\n",L);
    }
    return 0;
}