hdu 1348 Wall （Convex Hull）

http://acm.hdu.edu.cn/showproblem.php?pid=1348

　　简单二维凸包，直接套用andrew算法得到凸包后求周长，然后加上一个圆周即可。

　　做的过程中有一个小错误，就是之前已经cin >> n;了，做凸包的时候写成了int n = andrew(pt, n, ch); 从而导致segment fault。以后要注意避免这样的事再次发生。

代码如下：

#include <cstdio>
#include <cstring>
#include <cmath>
#include <vector>
#include <algorithm>
#include <iostream>

using namespace std;

struct Point {
    double x, y;
    Point() { x = y = 0.0;}
    Point(double x, double y) : x(x), y(y) {}
} ;
template <class T> T sqr(T x) { return x * x;}
inline double ptDis(Point a, Point b) { return sqrt(sqr(a.x - b.x) + sqr(a.y - b.y));}

typedef Point Vec;
Vec operator + (Vec a, Vec b) { return Vec(a.x + b.x, a.y + b.y);}
Vec operator - (Vec a, Vec b) { return Vec(a.x - b.x, a.y - b.y);}
Vec operator * (Vec a, double p) { return Vec(a.x * p, a.y * p);}
Vec operator / (Vec a, double p) { return Vec(a.x / p, a.y / p);}

const double EPS = 1e-8;
const double PI = acos(-1.0);
inline int sgn(double x) { return fabs(x) < EPS ? 0 : (x < 0 ? -1 : 1);}
bool operator < (Point a, Point b) { return a.x < b.x || a.x == b.x && a.y < b.y;}
bool operator == (Point a, Point b) { return !sgn(a.x - b.x) && !sgn(a.y - b.y);}

inline double dotDet(Vec a, Vec b) { return a.x * b.x + a.y * b.y;}
inline double crossDet(Vec a, Vec b) { return a.x * b.y - a.y * b.x;}
inline double crossDet(Point o, Point a, Point b) { return crossDet(a - o, b - o);}
inline double vecLen(Vec x) { return sqrt(dotDet(x, x));}

struct Line {
    Point s, t;
    Line() {}
    Line(Point s, Point t) : s(s), t(t) {}
} ;

int andrew(Point *pt, int n, Point *ch) {
//    cout << "sort!!" << n << endl;
    sort(pt, pt + n);
    int m = 0;
//    cout << "here??" << endl;
    for (int i = 0; i < n; i++) {
        while (m > 1 && crossDet(ch[m - 2], ch[m - 1], pt[i]) <= 0) m--;
        ch[m++] = pt[i];
    }
    int k = m;
    for (int i = n - 2; i >= 0; i--) {
        while (m > k && crossDet(ch[m - 2], ch[m - 1], pt[i]) <= 0) m--;
        ch[m++] = pt[i];
    }
    if (n > 1) m--;
    return m;
}

const int N = 1111;
Point pt[N], ch[N];

int main() {
//    freopen("in", "r", stdin);
    int T, n;
    double r;
    cin >> T;
    while (T--) {
        cin >> n >> r;
//        cout << n << ' ' << r << endl;
        for (int i = 0; i < n; i++) {
            cin >> pt[i].x >> pt[i].y;
        }
//        cout << "read?" << n << endl;
        n = andrew(pt, n, ch);
        ch[n] = ch[0];
        double sum = 0.0;
        for (int i = 0; i < n; i++) {
            sum += ptDis(ch[i], ch[i + 1]);
        }
        printf("%.0f\n", sum + 2.0 * PI * r);
        if (T) cout << endl;
    }
    return 0;
}

——written by Lyon