题解：
维护一个栈就好了，画画图就知道怎么维护了。。
这两天总写不对计算几何的水题。。真是郁闷。。
View Code
1 #include <iostream> 2 #include <cstring> 3 #include <cstdlib> 4 #include <cstdio> 5 #include <algorithm> 6 7 #define N 555555 8 #define EPS 1e-7 9 10 using namespace std; 11 12 struct PO 13 { 14 double x,y; 15 void prt() {printf("%lf %lf\n",x,y);} 16 }sk[N]; 17 18 struct LI 19 { 20 PO a,b; 21 int id; 22 double p,q; 23 void prt() {printf("%lf %lf %lf %lf\n",a.x,a.y,b.x,b.y);} 24 }li[N],stk[N]; 25 26 int n,top,tp,bh[N]; 27 28 inline PO operator +(PO a,PO b) 29 { 30 a.x+=b.x; a.y+=b.y; 31 return a; 32 } 33 34 inline PO operator -(PO a,PO b) 35 { 36 a.x-=b.x; a.y-=b.y; 37 return a; 38 } 39 40 inline PO operator *(PO a,double k) 41 { 42 a.x*=k; a.y*=k; 43 return a; 44 } 45 46 inline PO operator /(PO a,double k) 47 { 48 a.x/=k; a.y/=k; 49 return a; 50 } 51 52 inline int dc(double x) 53 { 54 if(x>EPS) return 1; 55 else if(x<-EPS) return -1; 56 return 0; 57 } 58 59 inline void read() 60 { 61 scanf("%d",&n); 62 double a,b; 63 for(int i=1;i<=n;i++) 64 { 65 scanf("%lf%lf",&a,&b); 66 li[i].a.x=0; li[i].a.y=b; 67 if(dc(a)==0) li[i].b.x=29.3278,li[i].b.y=b; 68 else li[i].b.x=29.3278,li[i].b.y=a*29.3278+b; 69 li[i].p=a; li[i].q=b; 70 li[i].id=i; 71 } 72 } 73 74 inline double cross(const PO &a,const PO &b,const PO &c) 75 { 76 return (b.x-a.x)*(c.y-a.y)-(c.x-a.x)*(b.y-a.y); 77 } 78 79 inline bool cmp(const LI &a,const LI &b) 80 { 81 if(dc(a.p-b.p)==0) return a.q>b.q; 82 return a.p<b.p; 83 } 84 85 inline PO getpoint(const PO &a,const PO &b,const PO &c,const PO &d) 86 { 87 double k1=cross(a,d,c),k2=cross(b,c,d); 88 PO tmp;tmp=b-a; 89 return a+tmp*k1/(k1+k2); 90 } 91 92 inline void go() 93 { 94 PO c; 95 sort(li+1,li+1+n,cmp); 96 top=tp=0; 97 for(int i=1;i<=n;i++) 98 { 99 if(dc(li[i].p-li[i-1].p)==0) continue; 100 while(tp>=1) 101 { 102 c=getpoint(li[i].a,li[i].b,stk[top].a,stk[top].b); 103 if(dc(c.x-sk[tp].x)<=0) top--,tp--; 104 else break; 105 } 106 stk[++top]=li[i]; 107 if(top==2&&tp==0) sk[++tp]=getpoint(stk[top].a,stk[top].b,stk[top-1].a,stk[top-1].b); 108 else if(tp>0) sk[++tp]=c; 109 } 110 for(int i=1;i<=top;i++) bh[i]=stk[i].id; 111 sort(bh+1,bh+1+top); 112 for(int i=1;i<top;i++) printf("%d ",bh[i]); 113 printf("%d",bh[top]); 114 } 115 116 int main() 117 { 118 read(),go(); 119 return 0; 120 }