uva1396 Most Distant Point from the Sea

二分答案，验证答案x的时候把所有边向内移动x的距离求半平面交，如果有解就说明可行。

#include<cstdio>#include<algorithm>#include<cmath>using namespace std;const double eps=1e-8;const int maxn=110;int cmp(double x){    if (x>=eps) return 1;    if (fabs(x)<eps) return 0;    return -1;}struct Vector{    double x,y;    Vector operator + (const Vector &v) const    {        return (Vector){x+v.x,y+v.y};    }    Vector operator - (const Vector &v) const    {        return (Vector){x-v.x,y-v.y};    }    Vector operator * (const double &k) const    {        return (Vector){x*k,y*k};    }    Vector operator / (const double &k) const    {        return (Vector){x/k,y/k};    }    double ang() const    {        return atan2(y,x);    }}a[maxn],f[maxn];typedef Vector Point;double dot(Vector v,Vector u){    return v.x*u.x+v.y*u.y;}double cross(Vector v,Vector u){    return v.x*u.y-v.y*u.x;}double len(Vector v){    return sqrt(dot(v,v));}Vector normal(Vector v){    Vector u=(Vector){-v.y,v.x};    return u/len(u);}struct Line{    Point p;    Vector v;    bool operator < (const Line &l) const    {        return cmp(v.ang()-l.v.ang())==-1;    }}g[maxn],que[maxn];bool onleft(Point p,Line l){    return cmp(cross(p-l.p,l.v))==-1;}Point intersection(Line l,Line m){    double t=cross(m.v,l.p-m.p)/cross(l.v,m.v);    return l.p+l.v*t;}int n;int check(double x){    int hd=1,tl=1;    Vector v;    for (int i=1;i<=n;i++)    {        v=a[i+1]-a[i];        g[i]=(Line){a[i]+normal(v)*x,v};    }    sort(g+1,g+n+1);    que[1]=g[1];    for (int i=2;i<=n;i++)    {        while (hd<tl&&!onleft(f[tl-1],g[i])) tl--;        while (hd<tl&&!onleft(f[hd],g[i])) hd++;        if (!(que[tl]<g[i])&&!(g[i]<que[tl]))        {            if (!onleft(f[tl-1],g[i])) que[tl]=g[i];        }        else que[++tl]=g[i];        if (hd<=tl) f[tl-1]=intersection(que[tl-1],que[tl]);    }    while (hd<tl&&!onleft(f[tl-1],que[hd])) tl--;    return tl-hd>=2;}void solve(){    double l,r,mid;    for (int i=1;i<=n;i++) scanf("%lf%lf",&a[i].x,&a[i].y);    a[n+1]=a[1];    l=0,r=5e4;    while (fabs(r-l)>1e-6)    {        mid=(l+r)/2;        if (check(mid)) l=mid;        else r=mid;    }    printf("%f\n",l);}int main(){    //freopen("a.in","r",stdin);    while (scanf("%d",&n)&&n) solve();}