POJ 1113 Wall(思维 计算几何 数学)

#include<cstdio>
#include<cmath>
#include<algorithm>
using namespace std;
const int MAXN = 1e5 + 10;
inline int read() {
    char c = getchar(); int x = 0, f = 1;
    while(c < '0' || c > '9') {if(c == '-') f = -1; c = getchar();}
    while(c >= '0' && c <= '9') x = x * 10 + c - '0', c = getchar();
    return x * f;
}
int N, top;
struct Point {
    double x, y;
    Point operator - (const Point &rhs) const {
        return {x - rhs.x, y - rhs.y};
    }
    Point operator + (const Point &rhs) const {
        return {x + rhs.x, y + rhs.y};
    }
    double operator ^ (const Point &rhs) const {
        return x * rhs.y - y * rhs.x;
    }
    bool operator < (const Point &rhs) const {
        return x == rhs.x ? y < rhs.y : x < rhs.x;
    }
}p[MAXN], q[MAXN];
template<typename A> A sqr(A x) {
    return x * x;
}
double dis(Point a, Point b) {
    return sqrt(sqr(a.x - b.x) + sqr(a.y - b.y));
}
void insert(Point now) {
    while(top > 1 && ((q[top] - q[top - 1]) ^ (now - q[top - 1])) < 0) top--;   
    q[++top] = now; 
}
int main() {
    N = read(); double L = read();
    for(int i = 1; i <= N; i++) p[i].x = read(), p[i].y = read();
    sort(p + 1, p + N + 1);
    q[top = 1] = p[1];
    for(int i = 2; i <= N; i++) insert(p[i]);
    for(int i = N - 1; i >= 1; i--) insert(p[i]);
    double ans = 0;
    for(int i = 1; i < top; i++) ans += dis(q[i], q[i + 1]);
    ans += 2 * acos(-1) * L + 0.5;
    printf("%d\n", (int) ans);
}