POJ 2187 Beauty Contest 旋转卡壳入门题

入门：http://blog.csdn.net/accry/article/details/6070626

#include<stdio.h>#include<string.h>#include<algorithm>using namespace std;#define maxn 50005struct point{    int x, y;}p[maxn], res[maxn<<1];int max(int a, int b){    return a > b ? a : b;}int xmult(point o, point a, point b){    return (a.x - o.x) * (b.y - o.y) - (a.y - o.y) * (b.x - o.x);}bool cmp(point a, point b){    return a.y < b.y || a.y == b.y && a.x < b.x;}int dis(point a, point b){    return (a.x - b.x) * (a.x - b.x) + (a.y - b.y) * (a.y - b.y);}int graham(int n, point *p){    int top = 1, i;        sort(p, p + n, cmp);    if(n == 0) return 0;    res[0] = p[0];    if(n == 1) return 1;    res[1] = p[1];    if(n == 2) return 2;    for(i = 3; i < n; i++)    {        while(top >= 1 && xmult(res[top-1], res[top], p[i]) <= 0)             top--;        res[++top] = p[i];    }    res[++top] = p[n-2];    int t = top;    for(i = n-3; i >= 0; i--)    {        while(top >= t && xmult(res[top-1], res[top], p[i]) <= 0)            top--;        res[++top] = p[i];    }    return top;}int rotating_calipers(int n, point *p){    int i, q = 1, ans = 0;    p[n] = p[0];    for(i = 0; i < n; i++)    {        while( xmult(p[i], p[i+1], p[q+1]) > xmult(p[i], p[i+1], p[q]))            q = (q + 1) % n;        ans = max(ans, max(dis(p[i], p[q]), dis(p[i+1], p[q+1]) ) );    }    return ans;}int main(){    int i, j;    int n;    while( ~scanf("%d", &n))    {        for(i = 0; i < n; i++)            scanf("%d%d", &p[i].x, &p[i].y);        int cnt = graham(n, p);        printf("%d\n", rotating_calipers(cnt, res) );    }    return 0;}