hdu2108
叉积（用坐标）能够判断边b是在边a的顺时针方向还是逆时针方向
代码如下：
1 #include <stdio.h> 2 #include <stdlib.h> 3 4 int main() 5 { 6 int n, i; 7 int coordinate[1000][2]; 8 int real( int coordinate[][2], int i ); 9 10 while( scanf( "%d", &n ), n ) 11 { 12 13 for( i = 0; i < n; i ++ )//逆时针读入坐标 14 { 15 scanf( "%d%d", &coordinate[i][0], &coordinate[i][1] ); 16 } 17 18 coordinate[n][0] = coordinate[0][0]; 19 coordinate[n][1] = coordinate[0][1]; 20 coordinate[n+1][0] = coordinate[1][0]; 21 coordinate[n+1][1] = coordinate[1][1]; 22 23 for( i = 0; i < n; i ++ ) 24 { 25 if( real( coordinate, i ) >= 0 )//大于0表示 为顺时针拐 26 { 27 break; 28 } 29 } 30 31 if( i == n ) 32 { 33 printf( "convex\n" ); 34 } 35 else 36 { 37 printf( "concave\n" ); 38 } 39 } 40 return 0; 41 } 42 int real( int coordinate[][2], int i ) 43 { 44 int x1, x2, y1, y2; 45 46 x1 = coordinate[i+2][0] - coordinate[i][0]; 47 x2 = coordinate[i+1][0] - coordinate[i][0]; 48 y1 = coordinate[i+2][1] - coordinate[i][1]; 49 y2 = coordinate[i+1][1] - coordinate[i][1]; 50 51 return - x2 * y1 + y2 * x1; 52 }
