极角排序（叉积）

#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){   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){   point *p1 = (point*)pp1, *p2 = (point*)pp2;   return CrossProd(list[0], *p1, *p2)*(-1);}int init(int n){   int i;   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]);         }   }    return n-1;}int main(){   int count = 0, n, num;   while(cin>>n){      if(0 == n){         break;      }      count++;      cout << "set" << count << ":\n";      num = init(n);      qsort(list+1, num, sizeof(point), comp);      for(int i = 0; i < n; i++){         printf("list[%d] = %d, list[%d] = %d\n", i, list[i].x, i, list[i].y);      }   }   return 0;}