北京G，计算几何，线段相交

bfs肯定没问题了，

不过中间的判断非常恶心

关键是三角形

出题人：这题很容易怀疑人生

提供几组数据(from zm)

20 00 22 0....21 10 11 0....20.5 0.50.5 1.51.5 0.5....

计算几何的边界好搞啊

#include <set>#include <cmath>#include <queue>#include <cstdio>#include <vector>#include <cstring>#include <iostream>#include <algorithm>typedef long long LL;using namespace std;const int N = 22;const int dx[] = {-1, -1, -1,  0,  0,  1,  1,  1};const int dy[] = {-1,  0,  1, -1,  1, -1,  0,  1};struct point{    double x, y;    point():x(0), y(0){}    point(double _x, double _y): x(_x),y(_y){}    inline void read(){        scanf("%lf%lf", &x, &y);    }    point operator + (const point &a) const {        return point(x+a.x, y+a.y);    }    point operator - (const point &a) const {        return point(x-a.x, y-a.y);    }    point operator * (const double &k) const {        return point(x * k, y * k);    }    double operator ^ (const point &a) const {  // det        return x*a.y - y*a.x;    }    double dis(const point &b) {        return (x-b.x)*(x-b.x) + (y-b.y)*(y-b.y);    }    inline void print(){        printf("point: (%.2lf, %.2lf)\n", x, y);    }    bool onSeg(point P1, point P2){        point Q = point(x, y);        if (min(P1.x,P2.x) > Q.x || Q.x > max(P1.x,P2.x)) return false;        if (min(P1.y,P2.y) > Q.y || Q.y > max(P1.y,P2.y)) return false;        double ans = fabs((Q - P1) ^ (Q - P2));        //printf("??? = %.4lf\n", ans);        if (fabs((Q - P1) ^ (Q - P2)) > 0) return false;        return true;    }} A, B, C;double s(point p, point a, point b){    return fabs((p - a) ^ (p - b));}bool inTra(point P, point A, point B, point C){    double s1 = s(P, A, B), s2 = s(P, B, C), s3 = s(P, C, A);    if (s1 == 0 || s2 == 0 || s3 == 0) return false;    double a =  s1 + s2 + s3;    double b = s(A, B, C);    //printf("%.2lf\n", a-b);    return fabs(a - b) == 0.;}bool segXseg(point p1, point p2, point p3, point p4){    if (p1.onSeg(p3, p4) || p2.onSeg(p3, p4)) return false;    if (p3.onSeg(p1, p2) || p4.onSeg(p1, p2)) return false;    //---------------------------------------------    if (max(p1.x, p2.x) < min(p3.x, p4.x)) return 0;    if (max(p1.y, p2.y) < min(p3.y, p4.y)) return 0;    if (max(p3.x, p4.x) < min(p1.x, p2.x)) return 0;    if (max(p3.y, p4.y) < min(p1.y, p2.y)) return 0;//ju xin shi yan    double x = (p3 - p1) ^ (p2 - p1);    double y = (p4 - p1) ^ (p2 - p1);    double z = (p1 - p3) ^ (p4 - p3);    double w = (p2 - p3) ^ (p4 - p3); //KuaLi    return x*y <= 0 && z*w <= 0;}bool xj(point p1, point p2, point A, point B, point C){  //xiangjiao    //for (double k = 0.01; k < 1; k += 0.01)        //if (inTra((p1*k + p2*(1-k)), A, B, C)) return true;    if (inTra(p1*0.01 + p2*0.99, A, B, C)) return true;    if (inTra(p1*0.99 + p2*0.01, A, B, C)) return true;    if (segXseg(p1, p2, A, B)) return true;    if (segXseg(p1, p2, A, C)) return true;    if (segXseg(p1, p2, C, B)) return true;    return false;}bool wea[N][N];  // weatherbool vis[N][N];int step[N][N];int bfs(const int &n, int sx, int sy){    if (!wea[sx][sy]) return -1;    memset(vis, false, sizeof vis);    vis[sx][sy] = true;    step[sx][sy] = 0;    queue <point> Q;    for (Q.push(point(sx, sy)); !Q.empty();){        point p = Q.front(); Q.pop();        if (p.x == n-1 && p.y == n-1) return step[n-1][n-1];         for (int i = 0; i < 8; i++){            int cx = (int)p.x + dx[i];            int cy = (int)p.y + dy[i];            if (cx < 0 || cx >= n|| cy < 0 || cy >= n) continue;            if (!wea[cx][cy] || vis[cx][cy]) continue;  //weather            if (xj(p, point(cx, cy), A, B, C)) continue;            step[cx][cy] = step[(int)p.x][(int)p.y] + 1;            vis[cx][cy] = true;            //printf("-----------(%d, %d)->(%d, %d), %d\n", (int)p.x, (int)p.y, cx, cy, step[cx][cy]);            Q.push(point(cx, cy));        }    }    return -1;}int main(){    //freopen("in.txt", "r", stdin);    char ch;    for (int n; ~scanf("%d", &n);){        A.read(); B.read(); C.read();        memset(wea, true, sizeof wea);        for (int i = 0; i < n; i ++)            for (int j = 0; j < n; j++) if (inTra(point(i, j), A, B, C)) {                //printf("%d, %d\n", i, j);                wea[i][j] = false;            }        scanf("\n");        for (int j = n-1; j >= 0; j--){            for (int i = 0; i < n; i++){                 scanf("%c", &ch);                if (ch == '#') wea[i][j] = false;            }            scanf("\n");        }        int ans = bfs(n, 0, 0);        printf("%d\n", ans);    }    return 0;}