多边形的面积

http://acm.hdu.edu.cn/vcontest/vtl/problem/showproblem/vtlid/1816/problemid/1005

View Code

   
     

    
      #include
    
      "
    
      iostream
    
      "
    
      

    
      using
    
       
    
      namespace
    
       std;

    
      int
    
       main()
{
 
    
      int
    
       t;
 
    
      int
    
       n,i;
 
    
      int
    
       x[
    
      101
    
      ],y[
    
      101
    
      ];
 cin
    
      >>
    
      t;
 
    
      while
    
      (t
    
      --
    
      )
 {
 
    
      double
    
       sum
    
      =
    
      0
    
      ;
 cin
    
      >>
    
      n;
 
    
      for
    
      (i
    
      =
    
      0
    
      ;i
    
      <
    
      n;i
    
      ++
    
      ) cin
    
      >>
    
      x[i]
    
      >>
    
      y[i];
 
    
      for
    
      (i
    
      =
    
      1
    
      ;i
    
      <
    
      n;i
    
      ++
    
      )
 {
 sum
    
      +=
    
      x[i]
    
      *
    
      y[i
    
      -
    
      1
    
      ]
    
      -
    
      x[i
    
      -
    
      1
    
      ]
    
      *
    
      y[i];
 }
 sum
    
      +=
    
      y[n
    
      -
    
      1
    
      ]
    
      *
    
      x[
    
      0
    
      ]
    
      -
    
      x[n
    
      -
    
      1
    
      ]
    
      *
    
      y[
    
      0
    
      ];
 
    
      if
    
      (sum
    
      <
    
      0
    
      ) sum
    
      =-
    
      sum;
 printf(
    
      "
    
      %.1lf\n
    
      "
    
      ,sum
    
      *
    
      0.5
    
      );
 }
 
    
      return
    
       
    
      0
    
      ;
}