UVA - 1606 Amphiphilic Carbon Molecules : 极角扫描法 利用叉积比较极角大小

#include <algorithm>
#include <iostream>
#include <sstream>
#include <utility>
#include <string>
#include <vector>
#include <queue>
#include <map>
#include <set>
#include <cstring>
#include <cstdio>
#include <cmath>
#define met(a,b) memset(a, b, sizeof(a));
#define IN freopen("in.txt", "r", stdin);
#define OT freopen("out.txt", "w", stdout);
using namespace std;
typedef long long LL;
typedef pair<int, int> PII;
const int maxn = 1e5 + 100;
const int INF = 0x7fffffff;
const int dir[5][2] = {0,0,-1,0,1,0,0,-1,0,1};
const LL MOD = 1e9 + 7;

struct pos{
int x, y, r; double rad;
bool operator < (const pos& b) const {
return rad < b.rad;
}
bool Left(const pos& b) const {
return x*b.y - y*b.x >= 0;
}
pos Repair(pos b) {
pos tmp;
tmp.x = x-b.x; tmp.y = y-b.y; tmp.r = r;
if(tmp.r == 1) tmp.x = -tmp.x, tmp.y = -tmp.y;
tmp.rad = atan2(tmp.y, tmp.x);
return tmp;
}
}p[maxn], q[maxn];
int n, qid, ans, cnt, cur;

int main() {
#ifdef _LOCAL
IN; //OT;
#endif // _LOCAL

while(scanf("%d", &n) == 1 && n) {
for(int i = 0; i < n; ++i) {
scanf("%d%d%d", &p[i].x, &p[i].y, &p[i].r);
}
if(n <= 3) { printf("%d\n", n); continue; }
ans = 0;
for(int i = 0; i < n; ++i) {
qid = 0;
for(int j = 0; j < n; ++j) if(i!=j) q[qid++] = p[j].Repair(p[i]);
sort(q, q+qid);

cnt = 2; cur = 0;
for(int j = 0; j < qid; ++j) {
if(cur == j) cur = (cur+1)%qid, ++cnt;
while(cur != j && q[j].Left(q[cur])) {
cur = (cur+1)%qid, ++cnt;
}
--cnt;
ans = max(ans, cnt);
}
}
printf("%d\n", ans);
}

return 0;
}