HDU2108-Shape of HDU

Calculate the Geometric Convex Hull problem. Use the cross product to determine whether all vertices meet the same direction of p1 (x1, y1), p2 (x2, y2), p3 (x3, y3 ); based on (x1-x3) * (y2-y3)-(x2-x3) * (x1-y3 );

 #include<map>  #include<set>  #include<list>  #include<cmath>  #include<ctime>  #include<deque>  #include<stack>  #include<bitset>  #include<cstdio>  #include<vector>  #include<cstdlib>  #include<cstring>  #include<iomanip>  #include<numeric>  #include<sstream>  #include<utility>  #include<iostream>  #include<algorithm>  #include<functional>    using namespace std ;  const int maxn = 10005 ;  struct node  {      int x , y ;   }edge[ maxn ] ;    int judge( int a , int b , int c )  {      return ( edge[ a ].x  - edge[ c ].x ) * ( edge[ b ].y - edge[ c ].y ) - ( edge[ b ].x - edge[ c ].x ) * ( edge[ a ].y - edge[ c ]. y ) ;   }    int main()  {      int n , flag , temp ;      while( scanf( "%d" , &n ) != EOF )      {          if( !n )          {              break ;          }          flag = 1 ;          for( int i = 0 ; i < n ; ++i )          {              scanf( "%d%d" , &edge[ i ].x , &edge[ i ].y ) ;          }           for( int i = 0 ; i < n ; ++i )          {              temp = judge( i % n , ( i + 1 ) % n , ( i + 2 ) % n ) ;              if( temp < 0 )              {                  flag = 0 ;                  break ;              }          }          if( flag )              printf( "convex\n" ) ;          else              printf( "concave\n" ) ;      }      return 0;  }