【HDOJ】3285 Convex Hull of Lattice Points

凸包模板题目。

/* 3285 */
#include <iostream>
#include <cstdio>
#include <cstring>
#include <cmath>
#include <algorithm>
using namespace std;

#define MAXN 55

typedef struct {
    int x, y;
} Point_t;

Point_t points[MAXN];
Point_t stack[MAXN];
int top;

void output(int);

double dist(Point_t a, Point_t b) {
    return sqrt(1.0*(a.x-b.x)*(a.x-b.x) + (a.y-b.y)*(a.y-b.y));
}

int dist2(Point_t a, Point_t b) {
    return (a.x-b.x)*(a.x-b.x) + (a.y-b.y)*(a.y-b.y);
}

int cross(Point_t a, Point_t b, Point_t c) {
    return (b.x-a.x)*(c.y-a.y) - (c.x-a.x)*(b.y-a.y);
}

bool comp(Point_t a, Point_t b) {
    int m = cross(points[0], a, b);
    return m>0 || (m==0 && dist2(points[0], a)<dist2(points[0], b));
}

void Graham(int n) {
    Point_t p;
    int i, j, k;
    
    p = points[0];
    k = 0;
    for (i=1; i<n; ++i) {
        if (points[i].x<p.x || (points[i].x==p.x && points[i].y<p.y)) {
            p = points[i];
            k = i;
        }
    }
    points[k] = points[0];
    points[0] = p;
    
    sort(points+1, points+n, comp);
    
    stack[0] = points[0];
    stack[1] = points[1];
    top = 1;
    
    for (i=2; i<n; ++i) {
        while(top && cross(stack[top-1], stack[top], points[i])<=0)
            --top;
        stack[++top] = points[i];
    }
}

int main() {
    int t, case_n;
    int n;
    int i, j, k;
    Point_t p;
    
#ifndef ONLINE_JUDGE
    freopen("data.in", "r", stdin);
#endif

    scanf("%d", &t);
    while (t--) {
        scanf("%d %d", &case_n, &n);
        for (i=0; i<n; ++i)
            scanf("%d %d", &points[i].x, &points[i].y);
        Graham(n);
        printf("%d %d
", case_n, top+1);
        p = stack[0];
        k = 0;
        for (i=1; i<=top; ++i) {
            if (stack[i].y>p.y || (stack[i].y==p.y && stack[i].x<p.x)) {
                p = stack[i];
                k = i;
            }
        }
        for (i=k; i>=0; --i)
            printf("%d %d
", stack[i].x, stack[i].y);
        for (i=top; i>k; --i)
            printf("%d %d
", stack[i].x, stack[i].y);
    }

    return 0;
}