求三角形内是否包含某点。就枚举四个点，懒得写凸包了。
#include <cstdio>#include <string>#include <cmath>#include <algorithm>using namespace std;#define _0 1e-10int N, A, B, C;typedef struct
...{
double x, y; char ch;
} Point;Point p[15];double inline dabs ( double d )...{ return d < _0 ? -d : d;}int cmp ( Point &a, Point &b )...{ if ( a.y == b.y ) return a.x < b.x; return a.y < b.y;}int bCmp ( Point &a, Point &b, Point &c )...{ //if ac is longer than ab double del = ( c.x - a.x ) * ( c.x - b.x ) + ( c.y - a.y ) * ( c.y - b.y ); if ( del > _0 ) return 1; if ( del < -_0 ) return -1; return 0;}int crossDet ( Point &a, Point &b, Point &c )...{ //if ac is on left side of ab double del = ( b.x - a.x ) * ( c.y - a.y ) - ( c.x - a.x ) * ( b.y - a.y ); if ( del > _0 ) return 1; if ( del < -_0 ) return -1; if ( bCmp ( a, b, c ) <= 0 ) return 0; return 2;}int inArea ( int a, int b, int c, int d )...{ int x1 = crossDet ( p[a], p[b], p[d] ); int x2 = crossDet ( p[b], p[c], p[d] ); int x3 = crossDet ( p[c], p[a], p[d] ); if ( !x1 || !x2 || !x3 ) return 1; if ( x1 == x2 && x2 == x3 ) return 1; return 0;}double calc ( int a, int b, int c )...{ double area = dabs ( 0.5 * ( ( p[c].y - p[a].y ) * ( p[b].x - p[a].x ) - ( p[b].y - p[a].y ) * ( p[c].x - p[a].x ) ) ); int d; for ( d = 0; d < N; d ++ )
...{ if ( d == a || d == b || d == c ) continue; if ( inArea ( a, b, c, d ) ) return 0.00;
} return area;}int main ()...{ //freopen ( "in.txt", "r", stdin ); while ( scanf ( "%d ", &N ) && N ) ...{ int i; for ( i = 0; i < N; i ++ ) scanf ( "%c %lf %lf ", &p[i].ch, &p[i].x, &p[i].y ); int a, b, c; double t, ans = -1; for ( a = 0; a < N; a ++ ) for ( b = a + 1; b < N; b ++ ) for ( c = b + 1; c < N; c ++ ) if ( ( t = calc ( a, b, c ) ) > ans ) ans = t, A = p[a].ch, B = p[b].ch, C = p[c].ch; printf ( "%c%c%c ", A, B, C ); } return 0;}