HDU 2036 改革春风吹满地 （计算几何）

#include <iostream>
#include <cstdio>
#include <cstring>
#include <queue>
#include <set>
#include <vector>
#include <algorithm>

using namespace std;
const int maxn = 100 + 5;
int x[maxn], y[maxn];

int cross(int x1, int y1, int x2, int y2){
    return x1 * y2 - x2 * y1;
}

int main(){
    int n;
    while(scanf("%d", &n) == 1 && n){
        for(int i = 0; i < n; ++i)  scanf("%d %d", &x[i], &y[i]);

        x[n] = x[0];  y[n] = y[0];
        double s = 0.0;
        for(int i = 0; i < n; ++i)  s += cross(x[i], y[i], x[i+1], y[i+1]);
        printf("%.1lf\n", s/2.0);
    }
}