Cleaning Shifts - POJ 2376 贪心

Cleaning Shifts

Time Limit: 1000MS Memory Limit: 65536KTotal Submissions: 11447 Accepted: 2982

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

3 101 73 66 10

Sample Output

2

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.

题意：一头牛可以工作相应的时间段，时候每天都有牛在工作，问至少需要几头牛。

思路：贪心，每次选取新增加的区间内的能够工作到最远时间的牛。

AC代码如下：

#include<cstdio>#include<cstring>#include<algorithm>using namespace std;struct node{    int s,e;}cow[25010];bool cmp(node a,node b){    return a.s<b.s;}int n,t,p;int find(int k){    int maxn=0;    while(cow[p].s<=k && p<=n)    {        maxn=max(maxn,cow[p].e);        p++;    }    return maxn;}int main(){    int i,j,k,l,num;    while(~scanf("%d%d",&n,&t))    {    for(i=1;i<=n;i++)       scanf("%d%d",&cow[i].s,&cow[i].e);    sort(cow+1,cow+1+n,cmp);    l=0;num=0,p=1;;    while(l<t)    {        k=find(l+1);        if(k<=l)          break;        else          l=k;        num++;    }    if(l<t)      printf("-1\n");    else      printf("%d\n",num);    }}