叉乘

HDU 2108 Shape of HDU　　判断凸凹边形

HDU 2036 改革春风吹满地　　计算任意多边形面积

// HDU 2036#include<iostream>#define MAX 10000using namespace std;int main(){int n, x[MAX], y[MAX];while(scanf("%d", &n) != EOF && n){int i;double area = 0;for(i = 0; i < n; i++)cin >> x[i] >> y[i];for(i = 0; i < n-1; i++)area += (x[i] * y[i+1] - x[i+1] *y[i]);area += (x[n-1] * y[0] - x[0] *y[n-1]);area /= 2;printf("%.1f\n", area);}}