根据旋转卡壳的思想，找到当前边的最远点。
确定i,j找到最远的k使 cross(i,j,k)最大，那么i,j+1时只需从k+1开始找即可 。
1 #include <iostream> 2 #include<cstdio> 3 #include<cstring> 4 #include<algorithm> 5 #include<stdlib.h> 6 #include<vector> 7 #include<cmath> 8 #include<queue> 9 #include<set> 10 using namespace std; 11 #define N 50010 12 #define LL long long 13 #define INF 0xfffffff 14 const double eps = 1e-8; 15 const double pi = acos(-1.0); 16 const double inf = ~0u>>2; 17 struct point 18 { 19 int x,y; 20 point(int x=0,int y=0):x(x),y(y) {} 21 } p[N],ch[N]; 22 typedef point pointt; 23 pointt operator - (point a,point b) 24 { 25 return point(a.x-b.x,a.y-b.y); 26 } 27 int dcmp(double x) 28 { 29 if(fabs(x)<eps) return 0; 30 return x<0?-1:1; 31 } 32 double cross(point a,point b) 33 { 34 return 1.0*a.x*b.y-1.0*a.y*b.x; 35 } 36 double getarea(point a,point b,point c) 37 { 38 return fabs(cross(b-a,c-a))/2; 39 } 40 double mul(point p0,point p1,point p2) 41 { 42 return cross(p1-p0,p2-p0); 43 } 44 double dis(point a) 45 { 46 return sqrt(1.0*a.x*a.x+a.y*a.y); 47 } 48 bool cmp(point a,point b) 49 { 50 if(dcmp(mul(p[0],a,b))==0) return dis(a-p[0])<dis(b-p[0]); 51 return dcmp(mul(p[0],a,b))>0; 52 } 53 int graham(int n) 54 { 55 int i,k=0,top; 56 point tmp; 57 for(i = 0; i< n; i++) 58 { 59 if(p[i].y<p[k].y||(p[i].y==p[k].y&&p[i].x<p[k].x)) 60 k = i; 61 } 62 if(k!=0) 63 swap(p[0],p[k]); 64 sort(p+1,p+n,cmp); 65 ch[0] = p[0]; 66 ch[1] = p[1]; 67 top = 1; 68 for(i = 2; i < n; i++) 69 { 70 while(top>0&&dcmp(mul(ch[top-1],ch[top],p[i]))<=0) 71 top--; 72 ch[++top] = p[i]; 73 } 74 return top; 75 } 76 int main() 77 { 78 int n,i,j,k,kk; 79 while(scanf("%d",&n)!=EOF) 80 { 81 if(n==-1) break; 82 for(i = 0; i < n; i++) 83 scanf("%d%d",&p[i].x,&p[i].y); 84 int top = graham(n); 85 double maxz=0,area; 86 ++top; 87 ch[top] = ch[0]; 88 for(i=0; i<top; ++i) 89 { 90 j=(i+1)%top; 91 k=(j+1)%top; 92 while(k!=i && getarea(ch[i],ch[j],ch[k])<getarea(ch[i],ch[j],ch[k+1])) 93 k=(k+1)%top; 94 maxz = max(maxz,getarea(ch[i],ch[j],ch[k])); 95 if(k == i) 96 continue; 97 kk=(k+1)%top; 98 while(j!=kk && k!=i) 99 { 100 area=getarea(ch[i],ch[j],ch[k]); 101 if(area>maxz) 102 area=maxz; 103 while(k!=i && getarea(ch[i],ch[j],ch[k])<getarea(ch[i],ch[j],ch[k+1])) 104 k=(k+1)%top; 105 maxz = max(maxz,getarea(ch[i],ch[j],ch[k])); 106 j=(j+1)%top; 107 } 108 } 109 printf("%.2f ",maxz); 110 111 }return 0; 112 }