裸的凸包。。。（和旋转卡壳有什么关系吗。。。蒟蒻求教T T）
话说忘了怎么写了。。。（我以前都是先做上凸壳再做下凸壳的说）
于是看了下hzwer的写法，用了向量的点积，方便多了，于是果断学习(Orz)了！
竟然比原作者要快T T
1 /************************************************************** 2 Problem: 1670 3 User: rausen 4 Language: C++ 5 Result: Accepted 6 Time:16 ms 7 Memory:900 kb 8 ****************************************************************/ 9 10 #include <cstdio> 11 #include <cmath> 12 #include <algorithm> 13 14 #define points P 15 using namespace std; 16 typedef long long ll; 17 const int N = 5005; 18 int n, top; 19 double ans; 20 struct points{ 21 int x, y; 22 }p[N], s[N]; 23 24 inline ll operator * (const P a, const P b){ 25 return a.x * b.y - a.y * b.x; 26 } 27 28 inline P operator - (const P a, const P b){ 29 P tmp; 30 tmp.x = a.x - b.x, tmp.y = a.y - b.y; 31 return tmp; 32 } 33 34 inline ll Sqr(const ll x){ 35 return x * x; 36 } 37 38 inline ll dis(const P a, const P b){ 39 return Sqr(a.x - b.x) + Sqr(a.y - b.y); 40 } 41 42 inline bool operator < (const P a, const P b){ 43 ll tmp = (a - p[1]) * (b - p[1]); 44 return tmp == 0 ? dis(p[1], a) < dis(p[1], b) : tmp > 0; 45 } 46 47 inline int read(){ 48 int x = 0, sgn = 1; 49 char ch = getchar(); 50 while (ch < '0' || ch > '9'){ 51 if (ch == '-') sgn = -1; 52 ch = getchar(); 53 } 54 while (ch >= '0' && ch <= '9'){ 55 x = x * 10 + ch - '0'; 56 ch = getchar(); 57 } 58 return sgn * x; 59 } 60 61 int work(){ 62 int k = 1; 63 for (int i = 2; i <= n; ++i) 64 if (p[i].y < p[k].y || (p[i].y == p[k].y && p[i].x < p[k].x)) k = i; 65 swap(p[1], p[k]); 66 sort(p + 2, p + n + 1); 67 top = 2, s[1] = p[1], s[2] = p[2]; 68 for (int i = 3; i <= n; ++i){ 69 while ((s[top] - s[top - 1]) * (p[i] - s[top]) <= 0) --top; 70 s[++top] = p[i]; 71 } 72 s[top + 1] = p[1]; 73 for (int i = 1; i <= top; ++i) 74 ans += sqrt(dis(s[i], s[i + 1])); 75 } 76 77 int main(){ 78 n = read(); 79 for (int i = 1; i <= n; ++i) 80 p[i].x = read(), p[i].y = read(); 81 work(); 82 printf("%.2lf ", ans); 83 }