POJ 3069

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

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Solution

题目大意：直线上有N个点，对应的点为Xi，对于任何一个点，以R为半径，它周围必须有被标记的点，在满足这个条件的情况下，求最少需要标记多少个点。
解题思路：
先对坐标进行升序排序。
1. 以a[0]为左端点，找满足a[i]>a[0]+r的第一个a[i],找到后以a[i-1]画圆。
2. 然后找到满足a[i-1]+r< a[j] 的第一个a[j],以它为左端点。
3. 重复上述步骤，直到j=n。

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Code

#include <stdio.h>#include <string.h>#include <iostream>#include <algorithm>using namespace std;#define N 1010int a[N];int main( ){    int r, n,j=1;    memset(a, -1, sizeof(a));    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);        int ans = 0;        int j = 0;        while (j<n)        {            int temp = a[j];            while (a[j] - temp <= r) j++;            temp = a[j - 1];            while (a[j] - temp <= r) j++;            ans++;        }        printf("%d\n", ans);    }    return 0;}