POJ - 2376 Cleaning Shifts（贪心）

最新推荐文章于 2020-05-10 12:04:23 发布

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该博客主要介绍了POJ 2376问题，即寻找覆盖1到T的最少区间个数。通过采用贪心策略，按区间开始时间排序，并维护当前最远结束时间，每次更新结束时间并计算最小区间数。建议使用for循环实现解题，避免潜在的while循环问题。这是一个涉及贪心算法和区间覆盖的经典问题。

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题目链接：http://poj.org/problem?id=2376点击打开链接

Cleaning Shifts

Time Limit: 1000MS		Memory Limit: 65536K
Total Submissions: 23408		Accepted: 5853

Description

Farmer John is assigning some of his N (1 <= N <= 25,000) cows to do some cleaning chores around the barn. He always wants to have one cow working on cleaning things up and has divided the day into T shifts (1 <= T <= 1,000,000), the first being shift 1 and the last being shift T.

Each cow is only available at some interval of times during the day for work on cleaning. Any cow that is selected for cleaning duty will work for the entirety of her interval.

Your job is to help Farmer John assign some cows to shifts so that (i) every shift has at least one cow assigned to it, and (ii) as few cows as possible are involved in cleaning. If it is not possible to assign a cow to each shift, print -1.

Input

* Line 1: Two space-separated integers: N and T

* Lines 2..N+1: Each line contains the start and end times of the interval during which a cow can work. A cow starts work at the start time and finishes after the end time.

Output

* Line 1: The minimum number of cows Farmer John needs to hire or -1 if it is not possible to assign a cow to each shift.

Sample Input

Sample Output

Hint

This problem has huge input data,use scanf() instead of cin to read data to avoid time limit exceed.

INPUT DETAILS:

There are 3 cows and 10 shifts. Cow #1 can work shifts 1..7, cow #2 can work shifts 3..6, and cow #3 can work shifts 6..10.

OUTPUT DETAILS:

By selecting cows #1 and #3, all shifts are covered. There is no way to cover all the shifts using fewer than 2 cows.

Source

USACO 2004 December Silver

给你一些区间求覆盖1～T最少区间个数

贪心思想以开始时间排序然后维护当前最远距离maxending 当前终点nowending

每次结束nowending将maxending的值赋给nowding 就能计算最小个数

注意这道题为开区间端点可以不重合

写法很多建议用for循环写

while写崩了。。

#include<iostream>
#include<algorithm>
#include<cstring>
#include<cstdio>
#include<queue>
#include<map>
#include<math.h>
#include<limits.h>
#include<vector>
using namespace std;
vector <pair<long long int,long long int > >s;
int main()
{
    long long int n,t;
    scanf("%lld%lld",&t,&n);
    for(int i=0;i<t;i++)
    {
        int mid1,mid2;
        scanf("%d%d",&mid1,&mid2);
        if(mid1>mid2)
            swap(mid1,mid2);
        s.push_back(make_pair(mid1, mid2));
    }
    sort(s.begin(),s.end());
    s.push_back(make_pair(INT_MAX, INT_MAX));
    int flag=0;
    int cnt=0;
    long long int maxending=0;
    long long int nowending=0;
    for(int i=0;i<s.size();i++)
    {
        if(s[i].first<=nowending+1)
        {
            if(s[i].second>maxending)
            {
                maxending=s[i].second;
                flag=1;
            }
            if(s[i+1].first>nowending+1&&flag)
            {
                nowending=maxending;
                cnt++;
                flag=0;
            }
        }
        
    }
    if(nowending<n)
        cout << "-1";
    else
        cout << cnt;
}