Graham

今天照着自己的理解写了Graham.

自己写错了很多次，所以先前看到的可能错了。。

const int maxn = 105;
struct Point
{
    float x,y;
}p[maxn],stack[maxn];

double xmult(Point p1,Point p2,Point p0)        //cross product 
{
    return (p1.x-p0.x)*(p2.y-p0.y)-(p1.y-p0.y)*(p2.x-p0.x);
}

double dis(Point p1,Point p2)        //distance
{
    return (p1.x-p2.x)*(p1.x-p2.x)+(p1.y-p2.y)*(p1.y-p2.y);
}

bool cmp(Point a,Point b)    //sort the element of the point besides p[0]
{
    double has;
    has=xmult(a,b,p[0]);
    if(has>0)        // a is the right of the b
        return 1;
    else if((has==0)&& (dis(a,p[0])<dis(b,p[0])))        //sort by distance
        return 1;
    else
        return 0;        //a is the left of th b
}

void swap(int r,int s)      
{
    Point temp;
    temp=p[r];
    p[r]=p[s];
    p[s]=temp;
}

//Graham
int Graham(int n)
{
    int i,top,k;
    Point ans;
    top=2;
    if(n<=3)        
    {
        for(i=0;i<n;i++)
            stack[i]=p[i];
        return n;
    }
    k=0;
    for(i=1;i<n;i++)        //find the left-under point
    if((p[i].y<p[k].y )|| ((p[i].y==p[k].y)&&(p[i].x<p[k].x)))
        k=i;
    ans=p[0];    p[0]=p[k];    p[k]=ans;        //set the left-under to the p[0]
    sort(p+1,p+n,cmp);    
    for(i=0;i<=top;i++)
        stack[i]=p[i];
    for(i=top+1;i<n;i++)
    {
        if((i+1<n)&&(xmult(p[i],p[i+1],p[0])==0))        //the point in a line
            continue;
        while(xmult(p[i],stack[top],stack[top-1])>=0)        //if the p[i] is the right of the stack[top]
                    top--;                                        //choose the p[i]
        stack[++top]=p[i];    
    }
    return ++top;
}