poj1113 Wall --凸包

原题链接： ​​http://poj.org/problem?id=1113​​

一：分析

二：AC代码

#define _CRT_SECURE_NO_DEPRECATE 

#include<iostream>
#include<cmath>
#include<algorithm>
using namespace std;

struct Point
{
  double x, y;

  Point operator-(Point & p)
  {
    Point t;
    t.x = x - p.x;
    t.y = y - p.y;
    return t;
  }

  double det(Point p)
  {
    return x*p.y - p.x*y;
  }

  double dist(Point & p)
  {
    return sqrt((x - p.x)*(x - p.x) + (y - p.y)*(y - p.y));
  }
};

bool cmp(Point & p1, Point & p2)
{
  if (p1.x != p2.x)
    return p1.x < p2.x;

  return p1.y < p2.y;
}

Point point[1005];
int convex[1005];
int N, L;

int getConvexHull()
{
  sort(point, point + N, cmp);
  int temp;
  int total = 0;

  for (int i = 0; i < N; i++)
  {
    while (total > 1 && (point[convex[total - 1]] - point[convex[total - 2]]).det(point[i] - point[convex[total - 1]]) <= 0)
      total--;
    convex[total++] = i;
  }

  temp = total;

  for (int i = N - 2; i >= 0; i--)
  {
    while (total > temp && (point[convex[total - 1]] - point[convex[total - 2]]).det(point[i] - point[convex[total - 1]]) <= 0)
      total--;
    convex[total++] = i;
  }

  return total;//返回组成凸包的点的个数，实际上多了一个，就是起点，所以组成凸包的点个数是total-1
}

int main()
{
  scanf("%d%d", &N, &L);

  for (int i = 0; i < N; i++)
    scanf("%lf%lf", &point[i].x, &point[i].y);

  int num = getConvexHull();
  double ans = 0.0;

  for (int i = 0; i < num - 1; i++)
    ans += point[convex[i]].dist(point[convex[i + 1]]);

  ans += 3.1415926 * 2 * L;

  printf("%.0lf\n", ans);

  return 0;
}