求凸包（Graham-Scan)

#include <iostream>
#include <cstdio>
#include <stdlib.h>
#include <cmath>
using namespace std;
const int MAX = 1000;
typedef struct point{
   int x, y;
   int flag;
}point;
point list[MAX];
int stack[MAX], top;

void swap(point &a, point &b){
   point t;
   t = a; a = b; b = t;
}

int CrossProd(point p0, point p1, point p2){//叉积
   return (p1.x-p0.x)*(p2.y-p0.y) - (p1.y-p0.y)*(p2.x-p0.x);
}

int comp(const void *pp1, const void *pp2){//比较函数
   point *p1 = (point*)pp1, *p2 = (point*)pp2;
   return CrossProd(list[0], *p1, *p2)*(-1);
}

void graham(int n){//用栈实现
   int i;
   if(1 == n){
      printf("(%d,%d)\n", list[0].x, list[0].y);
   }
   if(2 == n){
      printf("(%d,%d)(%d,%d)\n", list[0].x, list[0].y, list[1].x, list[1].y);
   }
   if(n > 2){
      for(i = 0; i <= 2; i++){
         stack[i] = i;
      }
      top = 2;
      for(i = 3; i <= n-1; i++){
         while(CrossProd(list[stack[top-1]], list[stack[top]], list[i])<=0){
            top--;
         }
         stack[++top] = i;
      }
      for(i = 0; i <= top; i++){
         printf("(%d,%d)\n", list[stack[i]].x, list[stack[i]].y);
      }
   }
}

void select(int n, int &num){//去掉角度相同的不部分点
   point p1, p2;
   int t, i, j;
   for(int i = 1; i <= n-2; i++){
      p1 = list[i];
      if(p1.flag){
         for(j = i + 1; j <= n-1; j++ ){
            p2 = list[j];
            if(p2.flag){
               t = CrossProd(list[0], p1, p2);
               if(0 == t){
                  if((p1.x-list[0].x)*(p1.x-list[0].x)+(p1.y-list[0].y)*(p1.y-list[0].y)
                     <(p2.x-list[0].x)*(p2.x-list[0].x)+(p2.y-list[0].y)*(p2.y-list[0].y)){
                        list[i].flag = 0;
                     }
                     else{
                        list[j].flag = 0;
                     }
               }
            }
         }
      }
   }
   i = 1;
   for(j = 1; j < n; j++){
      if(1 == list[j].flag){
         list[i++] = list[j];
      }
   }
   num = i - 1;//去掉list[0]余下的顶点数目
   qsort(list+1, num, sizeof(point), comp);
}

int init(int n){
   int i, num;
   for(i = 0; i < n; i++){
      cin >> list[i].x >> list[i].y;
      list[i].flag = 1;
      if((list[i].y < list[0].y)|| (list[i].y == list[0].y)
         && (list[i].x<list[0].x)){
            swap(list[0], list[i]);
         }
   }
   select(n, num);
   return num+1;//多边形的总点数
}

int main()
{
   int count = 0, n, num;
   while(cin>>n){
      if(0 == n){
         break;
      }
      cout << "set" << ++count << ":\n";
      num = init(n);
      graham(num);
   }
   return 0;
}
/*************
测试数据
13
0 0
70 -50
60 30
-30 -50
80 10
50 -10
0 -30
80 20
50 -60
90 -20
-30 -40
-10 -60
90 10
**********/