poj 1556 The Doors 最短路

    很水的一题，但是改了好久都没改对。

    原因是被定式思维害了，以为判断直线与点的位置只要返回个符号所以用int，但没想到负的double很小的时候变成int会成0，即丢失了原有符号。一直看了好几个小时才看出来。还是不够细致，这几天有点分心了。

/*author:jxylang:C/C++university:China,Xidian University**If you need to reprint,please indicate the source***/#include <iostream>#include <cstdio>#include <cstdlib>#include <cstring>#include <cmath>#include <queue>#define INF 1E9using namespace std;struct node{    double x,y;    node(double a,double b)    {        x=a;y=b;    }    node(){}};double dx[20],dy[20][4];double dis[80][80]={0};node pp[80];int n,cnt;double d(node a,node b){    return sqrt((a.x-b.x)*(a.x-b.x)+(a.y-b.y)*(a.y-b.y));}double p(node a,node b,double x,double y)//小于0，点c在直线ab下，反之上方{    return (b.x-a.x)*(y-a.y)-(x-a.x)*(b.y-a.y);}bool isok(node a,node b){    int i,j;    for(i=0;i<n&&dx[i]<=a.x;i++);    bool flag=1;    for(;i<n&&dx[i]<b.x;i++)    {        flag=0;        for(j=0;j<4;j+=2)           if(p(a,b,dx[i],dy[i][j])*p(a,b,dx[i],dy[i][j+1])<=0){flag=1;break;}        if(!flag)return 0;    }    return 1;}bool input(){    scanf("%d",&n);    if(n==-1)return 0;    int i,j;    cnt=0;    pp[cnt++]=node(0,5);    for(i=0;i<n;i++)    {        scanf("%lf",&dx[i]);        for(j=0;j<4;j++)        {            scanf("%lf",&dy[i][j]);            pp[cnt++]=node(dx[i],dy[i][j]);        }    }    pp[cnt++]=node(10,5);    for(i=0;i<cnt;i++)    {        for(j=i;j<cnt;j++)        {            dis[i][j]=dis[j][i]=INF;            if(pp[i].x==pp[j].x)continue;            if(!isok(pp[i],pp[j]))continue;            dis[i][j]=dis[j][i]=d(pp[i],pp[j]);       //     cout<<i<<" "<<j<<" "<<dis[i][j]<<endl;        }    }    return 1;}double dd[80];bool inq[80];double solve(){    int u,v;    memset(dd,127,sizeof(dd));    memset(inq,0,sizeof(inq));    queue<int> q;q.push(0);    dd[0]=0;    double t;    while(!q.empty())    {        v=q.front();q.pop();        inq[v]=0;        for(u=0;u<cnt;u++)           if(dd[u]>(t=dd[v]+dis[v][u]))           {               dd[u]=t;               if(inq[u])continue;               q.push(u);inq[u]=1;           }    }    return dd[cnt-1];}int main(){   while(input()) printf("%.2f\n",solve());}