poj1113Wall【凸包】

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Wall

Time Limit: 1000MS Memory Limit: 10000KTotal Submissions: 32437 Accepted: 11008

Description

Once upon a time there was a greedy King who ordered his chief Architect to build a wall around the King's castle. The King was so greedy, that he would not listen to his Architect's proposals to build a beautiful brick wall with a perfect shape and nice tall towers. Instead, he ordered to build the wall around the whole castle using the least amount of stone and labor, but demanded that the wall should not come closer to the castle than a certain distance. If the King finds that the Architect has used more resources to build the wall than it was absolutely necessary to satisfy those requirements, then the Architect will loose his head. Moreover, he demanded Architect to introduce at once a plan of the wall listing the exact amount of resources that are needed to build the wall. 

Your task is to help poor Architect to save his head, by writing a program that will find the minimum possible length of the wall that he could build around the castle to satisfy King's requirements. 

The task is somewhat simplified by the fact, that the King's castle has a polygonal shape and is situated on a flat ground. The Architect has already established a Cartesian coordinate system and has precisely measured the coordinates of all castle's vertices in feet.

Input

The first line of the input file contains two integer numbers N and L separated by a space. N (3 <= N <= 1000) is the number of vertices in the King's castle, and L (1 <= L <= 1000) is the minimal number of feet that King allows for the wall to come close to the castle. 

Next N lines describe coordinates of castle's vertices in a clockwise order. Each line contains two integer numbers Xi and Yi separated by a space (-10000 <= Xi, Yi <= 10000) that represent the coordinates of ith vertex. All vertices are different and the sides of the castle do not intersect anywhere except for vertices.

Output

Write to the output file the single number that represents the minimal possible length of the wall in feet that could be built around the castle to satisfy King's requirements. You must present the integer number of feet to the King, because the floating numbers are not invented yet. However, you must round the result in such a way, that it is accurate to 8 inches (1 foot is equal to 12 inches), since the King will not tolerate larger error in the estimates.

Sample Input

9 100200 400300 400300 300400 300400 400500 400500 200350 200200 200

Sample Output

1628

题意：在距城堡距离大于L的地方建围墙将城堡围起来求所要围墙的长度；

图片转自：http://blog.csdn.net/zhengnanlee/article/details/9633357；

所以所求长度如图所示为凸包周长加以l为半径的圆的周长。因为绕周长一周所以为一个正圆

#include<cstdio>#include<cstdlib>#include<cstring>#include<cmath>#include<algorithm>#define PI 3.1415926#define eps 1e-8using namespace std;struct point{double x,y;}A[1010],result[1010];bool equal(double n){return fabs(n-0)<eps;}double cp(point p1,point p2,point p3){return (p3.x-p1.x)*(p2.y-p1.y)-(p3.y-p1.y)*(p2.x-p1.x);}double dist(point a,point b){return sqrt((a.x-b.x)*(a.x-b.x)+(a.y-b.y)*(a.y-b.y));}bool cmp(point a,point b){double ans=cp(A[0],a,b);if(equal(ans))return dist(A[0],a)-dist(A[0],b)<=0;else return ans>0;}int main(){int n,i,j,l;while(scanf("%d%d",&n,&l)!=EOF){int pos=0;for(i=0;i<n;++i){scanf("%lf%lf",&A[i].x,&A[i].y);if(A[pos].y>=A[i].y){if(A[pos].y==A[i].y){if(A[pos].x>A[i].x)pos=i;}else pos=i;}}double S=0;point temp;temp=A[pos];A[pos]=A[0];A[0]=temp;sort(A+1,A+n,cmp);int top=1;result[0]=A[0];result[1]=A[1];for(i=2;i<n;++i){while(cp(result[top-1],result[top],A[i])<0)top--;result[++top]=A[i];}for(i=0;i<=top;++i){S+=dist(result[(i+1)%(top+1)],result[i]);}printf("%.0lf\n",S+2.0*PI*l);}return 0;}