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
", mid );
            }
            else r = mid;
        }

        printf( "%.2f
", ans );
    }
    return 0;
}