poj 3069

http://poj.org/problem?id=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, …, x_n of each troop (where 0 ≤ x_i ≤ 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.

题意：

给出几个点，给出半径，要求尽可能少标记几个点，实现对全部点的覆盖。

思路：

按照坐标排序，然后从左向右依次，第一个点是为标记的点找到最有段半径内的点，标记它，然后以标记点向右覆盖，找到第一个没有覆盖掉点，重复上述操作，知道结束。

#include<iostream>
#include<algorithm>
using namespace std;
int r,n;
int a[1003];
int solve(){
    sort(a,a+n);
    int i = 0;
    int ans = 0;
    while(i < n){
        int s = a[i++];
        while(i < n && a[i] <= s + r) i++;
        int troop = a[i-1];
        while(i < n && a[i] <= troop + r) i++;
        ans++;
    }
    return ans;
}
int main()
{
    ///input
    while(cin >> r >> n && (r != -1) && (n != -1)){
        for(int i = 0 ; i < n;i++)
            cin >> a[i];
         ///solve
        int ans = solve();
        ///output
        cout << ans << endl;
    }

    return 0;
}