hdu 2036 改革春风吹满地【多边形面积】

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题目链接：http://acm.hdu.edu.cn/showproblem.php?pid=2036
题意：求多边形面积
解析：把多边形拆成三角形，然后叉乘求三角形面积，然后求和

#include <cmath>#include <algorithm>#include <iostream>#include <cstdio>#include <vector>#include <cstring>using namespace std;const int maxn = 1000000+10;const double eps = 1e-5;struct point{    int x;    int y;    point() {}    point(int _x,int _y)    {        x = _x;        y = _y;    }}a[maxn];int x_mul(point p0,point p1,point p2){    return (p1.x-p0.x)*(p2.y-p0.y)-(p2.x-p0.x)*(p1.y-p0.y);}int area(int n){    int sum = 0;    for(int i=0;i<n;i++)        sum += x_mul(point(0,0),a[i],a[(i+1)%n]);    return sum;}int main(void){    int n;    while(scanf("%d",&n)&&n)    {        for(int i=0;i<n;i++)            scanf("%d %d",&a[i].x,&a[i].y);        double ans = area(n);        printf("%.1f\n",ans/2.0);    }    return 0;}

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