POJ 1755 Triathlon（线性规划の半平面交）

Description

Triathlon is an athletic contest consisting of three consecutive sections that should be completed as fast as possible as a whole. The first section is swimming, the second section is riding bicycle and the third one is running.

The speed of each contestant in all three sections is known. The judge can choose the length of each section arbitrarily provided that no section has zero length. As a result sometimes she could choose their lengths in such a way that some particular contestant would win the competition.
Input

The first line of the input file contains integer number N (1 <= N <= 100), denoting the number of contestants. Then N lines follow, each line contains three integers Vi, Ui and Wi (1 <= Vi, Ui, Wi <= 10000), separated by spaces, denoting the speed of ith contestant in each section.
Output

For every contestant write to the output file one line, that contains word "Yes" if the judge could choose the lengths of the sections in such a way that this particular contestant would win (i.e. she is the only one who would come first), or word "No" if this is impossible.

题目大意：铁人三项中，给出每个人玩铁人三项的速度，问能否通过调整这些比赛的距离（比如100米改成200米），使某个人获胜。
思路：设铁人三项的长度为x、y、z，x + y + z = 1，那么z = 1 - x - y。对于某个人cur，要满足对所有的人 i ，x / spd1[cur] + y / spd2[cur] + z / spd3[cur] < x / spd1[i] + y / spd2[i] + z / spd3[i]。
化简可得(1/spd1[i] - 1/spd1[cur] - 1/spd3[i] + 1/spd3[cur])*x + (1/spd2[i] - 1/spd2[cur] - 1/spd3[i] + 1/spd3[cur])*y + (1/spd3[i] - 1/spd3[cur]) > 0。
然后建立x+y<1和x>0,y>0，与上面的不等式，求是否存在可行域。求半平面交看有否可行域即可。

正在做模板，想办法把ax+by+c>0化成了两点式（见代码，要分类讨论），再做半平面交，我这种写法要EPS=1e-16才能过，丢的精度太多了。

代码（94MS）：

#include <cstdio>
#include <cstring>
#include <iostream>
#include <algorithm>
#include <cmath>
using namespace std;

const int MAXN = 110;
const double EPS = 1e-16;
const double PI = acos(-1.0);//3.14159265358979323846
const double INF = 1;

inline int sgn(double x) {
return (x > EPS) - (x < -EPS);
}

struct Point {
double x, y, ag;
Point() {}
Point(double x, double y): x(x), y(y) {}
void read() {
scanf("%lf%lf", &x, &y);
}
bool operator == (const Point &rhs) const {
return sgn(x - rhs.x) == 0 && sgn(y - rhs.y) == 0;
}
bool operator < (const Point &rhs) const {
if(y != rhs.y) return y < rhs.y;
return x < rhs.x;
}
Point operator + (const Point &rhs) const {
return Point(x + rhs.x, y + rhs.y);
}
Point operator - (const Point &rhs) const {
return Point(x - rhs.x, y - rhs.y);
}
Point operator * (const int &b) const {
return Point(x * b, y * b);
}
Point operator / (const int &b) const {
return Point(x / b, y / b);
}
double length() {
return sqrt(x * x + y * y);
}
Point unit() {
return *this / length();
}
void print() {
printf("%.10f %.10f\n", x, y);
}
};
typedef Point Vector;

double dist(const Point &a, const Point &b) {
return (a - b).length();
}

double cross(const Point &a, const Point &b) {
return a.x * b.y - a.y * b.x;
}
//ret >= 0 means turn left
double cross(const Point &sp, const Point &ed, const Point &op) {
return sgn(cross(sp - op, ed - op));
}

double area(const Point& a, const Point &b, const Point &c) {
return fabs(cross(a - c, b - c)) / 2;
}
//counter-clockwise
Point rotate(const Point &p, double angle, const Point &o = Point(0, 0)) {
Point t = p - o;
double x = t.x * cos(angle) - t.y * sin(angle);
double y = t.y * cos(angle) + t.x * sin(angle);
return Point(x, y) + o;
}

struct Seg {
Point st, ed;
double ag;
Seg() {}
Seg(Point st, Point ed): st(st), ed(ed) {}
void read() {
st.read(); ed.read();
}
void makeAg() {
ag = atan2(ed.y - st.y, ed.x - st.x);
}
};
typedef Seg Line;

//ax + by + c > 0
Line buildLine(double a, double b, double c) {
if(sgn(a) == 0 && sgn(b) == 0) return Line(Point(sgn(c) > 0 ? -1 : 1, INF), Point(0, INF));
if(sgn(a) == 0) return Line(Point(sgn(b), -c/b), Point(0, -c/b));
if(sgn(b) == 0) return Line(Point(-c/a, 0), Point(-c/a, sgn(a)));
if(b < 0) return Line(Point(0, -c/b), Point(1, -(a + c) / b));
else return Line(Point(1, -(a + c) / b), Point(0, -c/b));
}

void moveRight(Line &v, double r) {
double dx = v.ed.x - v.st.x, dy = v.ed.y - v.st.y;
dx = dx / dist(v.st, v.ed) * r;
dy = dy / dist(v.st, v.ed) * r;
v.st.x += dy; v.ed.x += dy;
v.st.y -= dx; v.ed.y -= dx;
}

bool isOnSeg(const Seg &s, const Point &p) {
return (p == s.st || p == s.ed) ||
(((p.x - s.st.x) * (p.x - s.ed.x) < 0 ||
(p.y - s.st.y) * (p.y - s.ed.y) < 0) &&
sgn(cross(s.ed, p, s.st) == 0));
}

bool isIntersected(const Point &s1, const Point &e1, const Point &s2, const Point &e2) {
return (max(s1.x, e1.x) >= min(s2.x, e2.x)) &&
(max(s2.x, e2.x) >= min(s1.x, e1.x)) &&
(max(s1.y, e1.y) >= min(s2.y, e2.y)) &&
(max(s2.y, e2.y) >= min(s1.y, e1.y)) &&
(cross(s2, e1, s1) * cross(e1, e2, s1) >= 0) &&
(cross(s1, e2, s2) * cross(e2, e1, s2) >= 0);
}

bool isIntersected(const Seg &a, const Seg &b) {
return isIntersected(a.st, a.ed, b.st, b.ed);
}

bool isParallel(const Seg &a, const Seg &b) {
return sgn(cross(a.ed - a.st, b.ed - b.st)) == 0;
}

//return Ax + By + C =0 's A, B, C
void Coefficient(const Line &L, double &A, double &B, double &C) {
A = L.ed.y - L.st.y;
B = L.st.x - L.ed.x;
C = L.ed.x * L.st.y - L.st.x * L.ed.y;
}
//point of intersection
Point operator * (const Line &a, const Line &b) {
double A1, B1, C1;
double A2, B2, C2;
Coefficient(a, A1, B1, C1);
Coefficient(b, A2, B2, C2);
Point I;
I.x = - (B2 * C1 - B1 * C2) / (A1 * B2 - A2 * B1);
I.y =   (A2 * C1 - A1 * C2) / (A1 * B2 - A2 * B1);
return I;
}

bool isEqual(const Line &a, const Line &b) {
double A1, B1, C1;
double A2, B2, C2;
Coefficient(a, A1, B1, C1);
Coefficient(b, A2, B2, C2);
return sgn(A1 * B2 - A2 * B1) == 0 && sgn(A1 * C2 - A2 * C1) == 0 && sgn(B1 * C2 - B2 * C1) == 0;
}

struct Poly {
int n;
Point p[MAXN];//p
= p[0]
void init(Point *pp, int nn) {
n = nn;
for(int i = 0; i < n; ++i) p[i] = pp[i];
p
= p[0];
}
double area() {
if(n < 3) return 0;
double s = p[0].y * (p[n - 1].x - p[1].x);
for(int i = 1; i < n; ++i)
s += p[i].y * (p[i - 1].x - p[i + 1].x);
return s / 2;
}
};

void Graham_scan(Point *p, int n, int *stk, int &top) {//stk[0] = stk[top]
sort(p, p + n);
top = 1;
stk[0] = 0; stk[1] = 1;
for(int i = 2; i < n; ++i) {
while(top && cross(p[i], p[stk[top]], p[stk[top - 1]]) >= 0) --top;
stk[++top] = i;
}
int len = top;
stk[++top] = n - 2;
for(int i = n - 3; i >= 0; --i) {
while(top != len && cross(p[i], p[stk[top]], p[stk[top - 1]]) >= 0) --top;
stk[++top] = i;
}
}
//use for half_planes_cross
bool cmpAg(const Line &a, const Line &b) {
if(sgn(a.ag - b.ag) == 0)
return sgn(cross(b.ed, a.st, b.st)) < 0;
return a.ag < b.ag;
}
//clockwise, plane is on the right
bool half_planes_cross(Line *v, int vn, Poly &res, Line *deq) {
int i, n;
sort(v, v + vn, cmpAg);
for(i = n = 1; i < vn; ++i) {
if(sgn(v[i].ag - v[i-1].ag) == 0) continue;
v[n++] = v[i];
}
int head = 0, tail = 1;
deq[0] = v[0], deq[1] = v[1];
for(i = 2; i < n; ++i) {
if(isParallel(deq[tail - 1], deq[tail]) || isParallel(deq[head], deq[head + 1]))
return false;
while(head < tail && sgn(cross(v[i].ed, deq[tail - 1] * deq[tail], v[i].st)) > 0)
--tail;
while(head < tail && sgn(cross(v[i].ed, deq[head] * deq[head + 1], v[i].st)) > 0)
++head;
deq[++tail] = v[i];
}
while(head < tail && sgn(cross(deq[head].ed, deq[tail - 1] * deq[tail], deq[head].st)) > 0)
--tail;
while(head < tail && sgn(cross(deq[tail].ed, deq[head] * deq[head + 1], deq[tail].st)) > 0)
++head;
if(tail <= head + 1) return false;
res.n = 0;
for(i = head; i < tail; ++i)
res.p[res.n++] = deq[i] * deq[i + 1];
res.p[res.n++] = deq[head] * deq[tail];
res.n = unique(res.p, res.p + res.n) - res.p;
res.p[res.n] = res.p[0];
return true;
}

/*******************************************************************************************/

Poly poly;
Line line[MAXN], deq[MAXN];
double a[MAXN], b[MAXN], c[MAXN];
char str[10];
int n, m;

double calc(double x, double y) {
return (y - x) / (x * y);
}

bool check(int cur) {
n = 0;
line[n++] = buildLine(-1, -1, INF); line[n - 1].makeAg();
line[n++] = buildLine(1, 0, 0); line[n - 1].makeAg();
line[n++] = buildLine(0, 1, 0); line[n - 1].makeAg();
for(int i = 0; i < m; ++i) {
if(i == cur) continue;
line[n++] = buildLine(calc(a[i], a[cur]) + calc(c[cur], c[i]), calc(b[i], b[cur]) + calc(c[cur], c[i]), INF * calc(c[i], c[cur]));
line[n - 1].makeAg();
//printf("%.10f %.10f\n", calc(a[i], a[cur]) + calc(c[cur], c[i]), calc(b[i], b[cur]) + calc(c[cur], c[i])), line[n - 1].st.print(), line[n - 1].ed.print();
}
bool flag = half_planes_cross(line, n, poly, deq);
return flag && sgn(poly.area());
}

int main() {
scanf("%d", &m);
for(int i = 0; i < m; ++i) scanf("%lf%lf%lf", &a[i], &b[i], &c[i]);
for(int i = 0; i < m; ++i)
if(check(i)) puts("Yes");
else puts("No");
}

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