ZOJ-1453

凸包第一题。。照搬了算法导论上的扫描法，计算几何这块还很菜，需要慢慢学习，本题应该算最最基础的题了，注意极角排序的比较函数，还有n为1或2的特殊情况的处理，因为这个WA了一次。。

#include<cstdio>#include<vector>#include<set>#include<algorithm>#include<cmath>using namespace std;namespace{pair<int, int> p;inline int cross(const pair<int, int> &p1, const pair<int, int> &p2){return p1.first * p2.second - p1.second * p2.first;}inline pair<int, int> vect(const pair<int, int> &p1,const pair<int, int> &p2){return make_pair(p2.first - p1.first, p2.second - p1.second);}bool cmp(const pair<int, int> &p1, const pair<int, int> &p2){return cross(vect(p, p2), vect(p, p1)) < 0;}double dist(const pair<int, int> &p1, const pair<int, int> &p2){return sqrt((p1.first - p2.first) * (p1.first - p2.first)+ (p1.second - p2.second) * (p1.second - p2.second));}}int main(){int n, x, y;set<pair<int, int> > S;vector<pair<int, int> > V;vector<pair<int, int> > Q;while (scanf("%d", &n), n){S.clear();V.clear();Q.clear();int miny = 99999, minx = 99999;for (int i = 0; i < n; i++){scanf("%d %d", &x, &y);S.insert(make_pair(x, y));if (y < miny){miny = y;minx = x;}else if (y == miny && x < minx)minx = x;}p = make_pair(minx, miny);S.erase(p);for (set<pair<int, int> >::iterator it = S.begin(); it != S.end(); it++)V.push_back(*it);if (n == 1){printf("0.00\n");continue;}else if (n == 2){printf("%.2lf\n", 2 * dist(p, V[0]));continue;}sort(V.begin(), V.end(), cmp);Q.push_back(p);Q.push_back(V[0]);Q.push_back(V[1]);for (size_t i = 2; i < V.size(); i++){pair<int, int> top = Q.back();pair<int, int> next_top = Q[Q.size() - 2];while (Q.size() > 1&& cross(vect(next_top, top), vect(next_top, V[i])) <= 0){Q.pop_back();top = Q.back();next_top = Q[Q.size() - 2];}Q.push_back(V[i]);}double sum = 0;for (size_t i = 1; i < Q.size(); i++)sum += dist(Q[i - 1], Q[i]);sum += dist(Q.front(), Q.back());printf("%.2lf\n", sum);}return 0;}