【代码】POJ 3207

// 题目来源：POJ 3207
// 题目大意：圆上顺序排列n个点，现要在一些点间连边，规定边只能在圆内或圆外，求有没有可能不相交
// 解决方法：连边只能在里面或者外面，这就是2-sat问题，直接缩点判断即可 
#include <cstdio>
#define o 5002

void link( int, int );
void tarjan( int );

int next[ o ], h[ o ], g[ o ], a[ o ], b[ o ], code[ o ], dfn[ o ], low[ o ], stack[ o ];
int n, m, t, top, index, cnt, i = 0, j;
bool ins[ o ];

int main( )
{
    freopen( "3207.in", "r", stdin );
    freopen( "3207.out", "w", stdout );
    scanf( "%d%d", &n, &m );
    for( int i = 0; i <= m-1; i++ )
    {
        scanf( "%d%d", &a[ i ], &b[ i ] );
        if( a[ i ] > b[ i ] )
        {
            int temp = a[ i ];
            a[ i ] = b[ i ];
            b[ i ] = temp;
        }
    }
    for( int i = 0; i <= m-1; i++ )
        for( int j = i + 1; j <= m-1; j++ )
            if( a[ i ] < a[ j ] && b[ i ] > a[ j ] && b[ i ] < b[ j ] || b[ i ] > b[ j ] && a[ i ] > a[ j ] && a[ i ] < b[ j ] )
            {
                link( i*2, j*2+1 );
                link( i*2+1, j*2 );
                link( j*2, i*2+1 );
                link( j*2+1, i*2 );
            }
    for( int i = 0; i <= 2*m-1; i++ )
        if( !dfn[ i ] ) tarjan( i );
    for( int i = 0; i <= 2*m-1; i++ )
        if( code[ i ] == code[ i^1 ] )
        {
            printf( "the evil panda is lying again" );
            return 0;
        }
    printf( "panda is telling the truth..." );
    return 0;
}

void link( int aa, int bb )
{
    next[ ++t ] = h[ aa ];
    h[ aa ] = t;
    g[ t ] = bb;
}

void tarjan( int i )
{
    int j;
    dfn[ i ] = low[ i ] = ++index;
    stack[ ++top ] = i;
    ins[ i ] = 1;
    for( int k = h[ i ]; k; k = next[ k ] )
    {
        j = g[ k ];
        if( !dfn[ j ] )
    	{
            tarjan( j );
            if( low[ j ] < low[ i ] ) low[ i ] = low[ j ];
        }
        else if( ins[ j ] )
            if( dfn[ j ] < low[ i ] ) low[ i ] = dfn[ j ];
    }
    if( low[ i ] == dfn[ i ] )
    {
        cnt++;
        do
        {
            j = stack[ top-- ];
            code[ j ] = cnt;
            ins[ j ] = 0;
        }
        while( i != j );
    }
}