poj 2187

求凸包后枚举凸包上的点

#include <cstdio>
#include <cstdlib>
#include <cmath>
#include <map>
#include <set>
#include <queue>
#include <stack>
#include <vector>
#include <sstream>
#include <string>
#include <cstring>
#include <algorithm>
#include <iostream>
#define maxn 100010
#define INF 0x7fffffff
#define inf 100000000
#define MOD 1000000007
#define ULL unsigned long long
#define LL long long

using namespace std;

const double ESP = 1e-10;

double add(double a, double b) {
    if(abs(a+b) < ESP * (abs(a) + abs(b))) return 0;
    return a+b;
}

struct P
{
    double x, y;

    P() {}

    P(double x, double y) : x(x), y(y) {}

    P operator - (P p) {
        return P(add(x, -p.x), add(y, -p.y));
    }

    P operator + (P p) {
        return P(add(x, p.x), add(y, p.y));
    }

    P operator * (double d) {
        return P(x*d, y*d);
    }

    double dot(P p) {
        return add(x*p.x, y*p.y);
    }

    double det(P p) {
        return add(x*p.y, - y*p.x);
    }
};

P ps[maxn];
int n;

bool cmp_x(const P& p, const P& q) {
    if(p.x != q.x) return p.x < q.x;
    return p.y < q.y;
}

vector<P> convex_full() {
    sort(ps, ps+n, cmp_x);
    int k = 0;
    vector<P> qs(n*2);
    for(int i = 0; i < n; ++ i) {
        while(k > 1 && (qs[k-1] - qs[k-2]).det(ps[i] - qs[k-1]) <= 0)
            -- k;
        qs[k++] = ps[i];
    }

    for(int i = n-2, t = k; i >= 0; -- i) {
        while(k > t && (qs[k-1] - qs[k-2]).det(ps[i] - qs[k-1]) <= 0)
            -- k;
        qs[k++] = ps[i];
    }
    qs.resize(k-1);
    return qs;
}

double dist(P p, P q) {
    return (p-q).dot(p-q);
}

void solve() {
    vector<P> qs = convex_full();
    // printf("%d\n", qs.size());
    double res = 0;
    for(int i = 0; i < (int)qs.size(); ++ i) {
        for(int j = 0; j < i; ++ j) {
            res = max(res, dist(qs[i], qs[j]));
        }
    }
    printf("%.0lf\n", res);
}

int main()
{
    while(scanf("%d", &n) == 1) {
        for(int i = 0; i < n; ++ i) {
            scanf("%lf%lf", &ps[i].x, &ps[i].y);
        }
        solve();
    }
    return 0;
}