poj 2007 Scrambled Polygon 凸包点排序逆时针输出

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求一多边形按逆时针顺序输出

用graham算法可以直接对其按逆时针排序

然后找到原点，把从原点开始把数组输出一遍

#include <stdio.h>  #include <cstring>  #include <cmath>  #include <algorithm>  using namespace std;    #define sqr(a) ((a) * (a))  #define dis(a, b) sqrt(sqr(a.x - b.x) + sqr(a.y - b.y))    const int MAXN = 110;  const double PI = acos(-1.0);    struct Point {      int x;      int y;      Point(double a = 0, double b = 0) : x(a), y(b) {}      friend bool operator < (const Point &l, const Point &r) {          return l.y < r.y || (l.y == r.y && l.x < r.x);      }  } p[MAXN], ch[MAXN];    double mult(Point a, Point b, Point o) {      return (a.x - o.x) * (b.y - o.y) >= (b.x - o.x) * (a.y - o.y);  }    int Graham(Point p[], int n, Point res[]) {      int top = 1;      sort(p, p + n);      if (n == 0) return 0;      res[0] = p[0];      if (n == 1) return 0;      res[1] = p[1];      if (n == 2) return 0;      res[2] = p[2];      int i;    for (i = 2; i < n; i++) {          while (top && (mult(p[i], res[top], res[top - 1])))              top--;          res[++top] = p[i];      }      int len = top;      res[++top] = p[n - 2];      for (i = n - 3; i >= 0; i--) {          while (top != len && (mult(p[i], res[top], res[top - 1])))              top--;          res[++top] = p[i];      }      return top;  }    int n;    int main() {  #ifndef ONLINE_JUDGEfreopen("in.txt","r",stdin);#endif    while (scanf("%d%d", &p[n].x, &p[n].y) != EOF)          n++;      n = Graham(p, n, ch);      int t,i;      for (i = 0; i < n; i++)          if (ch[i].x == 0 && ch[i].y == 0) {              t = i;              break;          }        for (i = t; i < n; i++)          printf("(%d,%d)\n", ch[i].x, ch[i].y);      for (i = 0; i < t; i++)          printf("(%d,%d)\n", ch[i].x, ch[i].y);      return 0;  }

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