poj 3069 Saruman's Army 【贪心】

Saruman's Army

Time Limit: 1000MS Memory Limit: 65536KTotal Submissions: 5770 Accepted: 2952

Description

Saruman the White must lead his army along a straight path from Isengard to Helm’s Deep. To keep track of his forces, Saruman distributes seeing stones, known as palantirs, among the troops. Each palantir has a maximum effective range ofR units, and must be carried by some troop in the army (i.e., palantirs are not allowed to “free float” in mid-air). Help Saruman take control of Middle Earth by determining the minimum number of palantirs needed for Saruman to ensure that each of his minions is within R units of some palantir.

Input

The input test file will contain multiple cases. Each test case begins with a single line containing an integerR, the maximum effective range of all palantirs (where 0 ≤ R ≤ 1000), and an integern, the number of troops in Saruman’s army (where 1 ≤ n ≤ 1000). The next line contains n integers, indicating the positionsx₁, …, x_n of each troop (where 0 ≤ x_i ≤ 1000). The end-of-file is marked by a test case withR = n = −1.

Output

For each test case, print a single integer indicating the minimum number of palantirs needed.

Sample Input

0 310 20 2010 770 30 1 7 15 20 50-1 -1

Sample Output

24

Hint

In the first test case, Saruman may place a palantir at positions 10 and 20. Here, note that a single palantir with range 0 can cover both of the troops at position 20.

In the second test case, Saruman can place palantirs at position 7 (covering troops at 1, 7, and 15), position 20 (covering positions 20 and 30), position 50, and position 70. Here, note that palantirs must be distributed among troops and are not allowed to “free float.” Thus, Saruman cannot place a palantir at position 60 to cover the troops at positions 50 and 70.

Source

Stanford Local 2006

 

思路：

 

          先将输入的坐标进行排序，然后从左往右找一个半径为R的圆的圆心（也就是第一个循环），然后找终点，找到终点也就是下次循环的起点，一直这样循环，知道终点的下标大于等于n为止！

 

代码：

 

#include <stdio.h>#include <string.h>#include <algorithm>using namespace std;int r,n;int a[1005];int main(){while(scanf("%d%d",&r,&n)&&(r!=-1||n!=-1)){for(int i=0;i<n;i++){scanf("%d",&a[i]);}sort(a,a+n);//坐标从小到大排序 int i=0,cnt=0;//i用来保存最左边的下标 while(i<n){int s=a[i++];//s用来保存还没有被包围的第一个点的坐标 while(i<n&&a[i]<=s+r)i++;//用来找要标记的点 int p=a[i-1];while(i<n&&a[i]<=p+r)i++;//用来找最末尾的点，也就是下一个点的起点 cnt++;//标记的点的个数 }printf("%d\n",cnt);}return 0;}