多边形的面积

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

   
   
   

    
    
    #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
    
    
    ;
}