回忆了一下，半平面交，整理了一下模版。
1 #include <cstdio> 2 #include <cstring> 3 #include <string> 4 #include <cmath> 5 #include <algorithm> 6 using namespace std; 7 #define eps 1e-8 8 #define N 2001 9 struct point 10 { 11 double x,y; 12 }p[N],temp[N],pre[N]; 13 int n,m; 14 double a,b,c; 15 void getline(point x,point y) 16 { 17 a = y.y - x.y; 18 b = x.x - y.x; 19 c = y.x * x.y - x.x * y.y; 20 } 21 point intersect(point x,point y) //获取直线ax+by+c==0 和点x和y所连直线的交点 22 { 23 double u = fabs(a*x.x+b*x.y+c); 24 double v = fabs(a*y.x+b*y.y+c); 25 point ans; 26 ans.x = (x.x*v+y.x*u)/(u+v); 27 ans.y = (x.y*v+y.y*u)/(u+v); 28 return ans; 29 } 30 void cut()//坐标顺时针给出，注释为逆时针 31 { 32 int num = 0,i; 33 for(i = 1;i <= m;i ++) 34 { 35 if(a*p[i].x + b*p[i].y + c >= -eps)//<= eps 36 { 37 temp[++num] = p[i]; 38 } 39 else 40 { 41 if(a*p[i-1].x + b*p[i-1].y + c > eps)//< -eps 42 temp[++num] = intersect(p[i],p[i-1]); 43 if(a*p[i+1].x + b*p[i+1].y + c > eps) 44 temp[++num] = intersect(p[i],p[i+1]);//< -eps 45 } 46 } 47 for(i = 1;i <= num;i ++) 48 p[i] = temp[i]; 49 p[0] = p[num]; 50 p[num+1] = p[1]; 51 m = num; 52 } 53 void fun() 54 { 55 int i; 56 m = n; 57 for(i = 1;i <= n;i ++) 58 { 59 getline(pre[i],pre[i+1]); 60 cut(); 61 } 62 } 63 int main() 64 { 65 int i,t; 66 scanf("%d",&t); 67 while(t--) 68 { 69 scanf("%d",&n); 70 for(i = 1;i <= n;i ++) 71 { 72 scanf("%lf%lf",&pre[i].x,&pre[i].y); 73 p[i] = pre[i]; 74 } 75 pre[n+1] = pre[1]; 76 p[n+1] = p[1]; 77 p[0] = p[n]; 78 fun(); 79 double s1,s2; 80 s1 = s2 = 0; 81 for(i = 0;i < m;i ++) 82 { 83 s1 += p[(i+1)%n].y*p[i].x; 84 s2 += p[(i+1)%n].x*p[i].y; 85 } 86 printf("%.2f ",fabs(s2-s1)/2+eps); 87 } 88 return 0; 89 }