graham模板整理

感觉以前写的graham代码风格太差了，然后又重新整理了一份

struct Node{    double x,y;    int pos;    double val;}a[maxn],s;double across(Node x,Node y,Node z)//z在x,y连线的顺时针为负{    return (x.x - y.x) * (z.y - y.y) - (x.y - y.y) * (z.x - y.x);}bool cmp(Node x,Node y){    if(x.val == y.val)return d2(x,s) < d2(y,s);    return x.val > y.val;}void Init(){for(int i = 1;i <= n;i ++){     scanf("%lf%lf",&a[i].x,&a[i].y);a[i].pos = i;     if(a[i].y < s.y || (a[i].y == s.y && a[i].x < s.x))s = a[i];}for(int i = 1;i <= n;i ++)if(a[i].x == s.x && a[i].y == s.y)a[i].val = 1;else a[i].val = get(a[i],s);sort(a + 1,a + n + 1,cmp);}void graham()  {      Node st[maxn];      int len = 0;      st[++ len] = a[1];      if(n>= 2)      st[++ len] = a[2];      if(n >= 3)      st[++ len] = a[3];      for(int i = 4; i <= n; i ++)      {          while(across(st[len],st[len],a[i]) <= 0)//< 0是可以允许共线，<= 0是不允许          {              len --;          }          st[++ len] = a[i];      }  //周长    double ans = 0;    for(int i = 1; i <= len - 1; i ++)      {          ans += distances(st[i],st[i + 1]);      }      ans += distances(st[1],st[len]) ;//面积:double anss = 0;      for(int i = 1; i <= len - 1; i ++)      {          anss += (st[i].x * st[i + 1].y - st[i + 1].x * st[i].y) / 2;      }    }