HDU 3622 Bomb Game 二分+2-SAT

二分爆炸半径R，判断是否可行，如果可行，半径可以继续增加。

之前建图有误，结果一直不对。

 

#include <cstdio>

#include <cstring>

#include <cstdlib>

#include <cmath>

#include <algorithm>

#include <vector>



using namespace std;



const int MAXN = 222;

const double eps = 1e-5;



struct Point

{

    double x, y;

    Point( double x = 0.0, double y = 0.0 ): x(x), y(y) { }

    void readPoint()

    {

        scanf( "%lf%lf", &x, &y );

        return;

    }

};



struct TwoSAT

{

    int n;

    vector<int> G[ MAXN << 1 ];

    bool mark[MAXN << 1];

    int S[MAXN << 1], top;



    bool DFS( int x )

    {

        if ( mark[x^1] ) return false;

        if ( mark[x] )   return true;

        mark[x] = true;

        S[ ++top ] = x;

        int sz = G[x].size();

        for ( int i = 0; i < sz; ++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, false, sizeof(mark) );

        return;

    }



    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);

        return;

    }



    bool solved()

    {

        for ( int i = 0; i < ( n * 2 ); i += 2 )

            if ( !mark[i] && !mark[i + 1] )

            {

                top = 0;

                if( !DFS(i) )

                {

                    while ( top > 0 ) mark[ S[top--] ] = false;

                    if ( !DFS(i + 1) ) return false;

                }

            }

        return true;

    }

};



int N;

Point P[MAXN][2];

TwoSAT Graph;

double dist[110][2][110][2];



double dcmp( double a )

{

    if ( fabs(a) < eps ) return 0;

    return a < 0 ? -1 : 1;

}



double dis( Point a, Point b )

{

    return sqrt( (a.x - b.x)*(a.x - b.x) + (a.y - b.y)*(a.y - b.y) );

}



bool check( double mid )

{

    Graph.init( N );

    mid *= 2.0;

    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 ( dcmp( dist[i][a][j][b] - mid ) < 0 )

                    {

                        Graph.add_clause( i, a ^ 1, j,  b ^ 1 );

                    }

                }



    return Graph.solved();

}



int main()

{

    while ( scanf( "%d", &N ) == 1 )

    {

        for ( int i = 0; i < N; ++i )

        {

            P[i][0].readPoint();

            P[i][1].readPoint();

        }



        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 )

                        dist[i][a][j][b] = dist[j][b][i][a] = dis( P[i][a], P[j][b] );



        double l = 0.0, r = dis( Point( -10000.0, -10000.0 ), Point( 10000.0, 10000.0 ) );

        double mid, ans;

        while ( r - l > eps )

        {

            mid = ( l + r ) / 2.0;

            if ( check(mid) )

            {

                ans = mid;

                l = mid;

                //printf( "mid = %.2f\n", mid );

            }

            else r = mid;

        }



        printf( "%.2f\n", ans );

    }

    return 0;

}