Saruman's Army

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 of R 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 integer R, the maximum effective range of all palantirs (where 0 ≤ R ≤ 1000), and an integer n, the number of troops in Saruman’s army (where 1 ≤ n ≤ 1000). The next line contains n integers, indicating the positions x1, …, xn of each troop (where 0 ≤ xi ≤ 1000). The end-of-file is marked by a test case with R = n = −1.
Output
For each test case, print a single integer indicating the minimum number of palantirs needed.
Sample Input
0 3
10 20 20
10 7
70 30 1 7 15 20 50
-1 -1
Sample Output
2 4

思路：贪心法。我们从最左边开始考虑。对于这个点，到距其r以内的区域内必须带有标记的点。带有标记的这个点显然在此点的右侧，我们应该从最左边的点开始加上标记，因为更左的区域没有覆盖的意义，所以尽可能覆盖更靠右的点。p点就是被标记的点，剩下的部分同样这么处理，找到其右侧r距离以内最远的点添加标记。

import java.util.Arrays;import java.util.Scanner;public class SarumanArmy {    public static void main(String[] args){        Scanner in=new Scanner(System.in);        while (in.hasNext()) {            int r=in.nextInt();            int n=in.nextInt();            int arr[]=new int[n];            for (int i = 0; i < arr.length; i++) {                arr[i]=in.nextInt();            }            Arrays.sort(arr);            int i=0,ans=0;            while (i<n) {                int s=arr[i++];                while(i<n && arr[i]<=s+r) {                    i++;                }                int p=arr[i-1];                while (i<n && arr[i]<=p+r) {                    i++;                }                ans++;            }            System.out.println(ans);        }        // TODO Auto-generated method stub    }}