poj 3069 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, …, 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.

Sample Input

0 3
10 20 20
10 7
70 30 1 7 15 20 50
-1 -1

Sample Output

2
4

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.

题目的大意

给你n个点，然后让你标记其中尽可能少的点，使得n个点都处于被标记点左右不超过R的区间内

即用一个最小的标记数目以达到覆盖整个结点的目的，求这个数目是多少？

利用贪心法求解

开始的代码不知道为什么一直错误，如下

#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <iostream>
#include <algorithm>
using namespace std;
int n ,r;
int troop[1002];

int cmp(const void *a, const void *b) {
    int at = *(int *)a;
    int bt = *(int *)b;
    return at - bt;
}
int main(int argc, char const *argv[])
{
    //freopen("input.txt","r",stdin);
    while(scanf("%d %d",&r,&n) != EOF && (r!=-1 && n != -1)) {
        for(int i = 0; i < n; i++) {
            scanf("%d",&troop[i]);
        }
        qsort(troop, n, sizeof(int), cmp);
        int cnt = 0;
        int halfCover = troop[0] + r;
        int cover = 0;
        int i = 1;
        int j = 0;
        while(cover < troop[n-1]) {
            while(i < n && troop[i] <= halfCover ) {
                j = i;
                i++;
            }
            cover = troop[j] + r;
            cnt++;
            while(i < n && troop[i] <= cover) {
                i++;
            }
            if(i == n) {
                break;
            }
            halfCover = troop[i] + r;
        }
        printf("%d
",cnt);
    }
    return 0;
}

改成这样就正确了

#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <iostream>
#include <algorithm>
using namespace std;
int n ,r;
int troop[1002];

int cmp(const void *a, const void *b) {
    int at = *(int *)a;
    int bt = *(int *)b;
    return at - bt;
}
int main(int argc, char const *argv[])
{
    //freopen("input.txt","r",stdin);
    while(scanf("%d %d",&r,&n) != EOF && (r!=-1 && n != -1)) {
        for(int i = 0; i < n; i++) {
            scanf("%d",&troop[i]);
        }
        qsort(troop, n, sizeof(int), cmp);
        int cnt = 0;
        int i = 0;
        while(i < n) {
            int halfCover = troop[i] + r;
            while(i < n && troop[i] <=  halfCover) {
                i++;
            }
            int cover = troop[i-1] + r;
            cnt++;
            while(i < n && troop[i] <= cover) {
                i++;
            }
        }
        printf("%d
",cnt);
    }
    return 0;
}