Triangle POJ

Triangle

POJ - 2079

求最大三角形

旋转卡壳

#include <cstdio>
#include <cstring>
#include <algorithm>
#include <iostream>
#include <cmath>
using namespace std;
const double eps = 1e-8;
const int inf = 0x3f3f3f3f;
const int maxn = 50010;

struct Point{
    double x, y;
    Point(double x = 0, double y = 0): x(x), y(y){}
};
typedef Point Vector;

Vector operator - (Point a, Point b) {
    return Vector(a.x - b.x, a.y - b.y);
}
Vector operator + (Vector a, Vector b) {
    return Vector(a.x + b.x, a.y + b.y);
}
Vector operator * (Vector a, double p) {
    return Vector(a.x * p, a.y * p);
}
Vector operator / (Vector a, double p) {
    return Vector(a.x / p, a.y / p);
}
bool operator < (Point a, Point b) {
    return a.x < b.x || a.x == b.x && a.y < b.y;
}
int dcmp(double x) {
    if(fabs(x) < eps) return 0;
    return x < 0 ? -1 : 1;
}

bool operator == (Point a, Point b) {
    return dcmp(a.x - b.x) == 0 && dcmp(a.y - b.y) == 0;
}
double Dot(Vector a, Vector b) {
    return a.x * b.x + a.y * b.y;
}
double Cross(Vector a, Vector b) {
    return a.x * b.y - a.y * b.x;
}
double Length(Vector a) {
    return sqrt(Dot(a, a));
}
double Angle(Vector a) {
    return atan2(a.y, a.x);
}

int ConvexHull(Point *p, int n, Point *ch) {
    sort(p, p+n);
    int m = 0;
    for(int i = 0; i < n; i++){
        while(m > 1 && Cross(ch[m-1] - ch[m-2], p[i] - ch[m-2]) <= 0) m--;
        ch[m++] = p[i];
    }
    int k = m;
    for(int i = n-2; i >= 0; i--){
        while(m > k && Cross(ch[m-1] - ch[m-2], p[i] - ch[m-2]) <= 0) m--;
        ch[m++] = p[i];
    }
    if(n > 1) m--;
    return m;
}
Point p[maxn], ch[maxn];
int vis[maxn];

double RotateCalipers(Point *p, int n){
    if(n < 3) return 0;
    p[n] = p[0];
    int i = 0, j = 1, k = 2;
    int a, b, c;
    double area = -inf;
    memset(vis, 0, sizeof(vis));
    while(!vis[2]){
        a = i, b = j, c = k;
        while(Cross(p[j] - p[i], p[k] - p[i]) < Cross(p[j] - p[i], p[k+1] - p[i])) k = (k+1) % n, vis[k] = 1;
        while(Cross(p[j] - p[i], p[k] - p[i]) < Cross(p[j+1] - p[i], p[k] - p[i])) j = (j+1) % n;
        while(Cross(p[j] - p[i], p[k] - p[i]) < Cross(p[j] - p[i+1], p[k] - p[i+1])) i = (i+1) % n;
        area = max(area, Cross(p[j] - p[i], p[k]  - p[i]));
        if(a==i && b==j && c==k) {
            k = (k+1)%n;
            vis[k] = 1;
        }
    }
    return area / 2;
}

int main(){
    //freopen("in.txt", "r", stdin);
    int n;
    while(scanf("%d", &n) && ~n){
        for(int i = 0; i < n; i++){
            scanf("%lf %lf", &p[i].x, &p[i].y);
        }
        int sz = ConvexHull(p, n , ch);
        printf("%.2f
", RotateCalipers(ch, sz));
    }
    return 0;
}