poj 2376 最小区间覆盖问题

http://poj.org/problem?id=2376

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 10
1 7
3 6
6 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.

所有的活必须在1~T天里完成，每头牛可以 再a[i].begain~a[i].end时间内干活，现将活安排给牛去做，尽量安排最少的牛去完成。

贪心问题的典型例子，最小区间覆盖问题：以每头牛开始的时间为标准，从小到大为牛排序。然后，对于没有找过的牛，选取begin小于Max且end大于Max的最大区间（牛），直到end>=T为止。

代码：

#include 
#include 
#include 
#include 
using namespace std;
struct A
{
    int left,right;
};
int cmp(const A &a,const A &b)
{
    if(a.left==b.left)
      return a.right> a[i].left>>a[i].right;
            if(a[i].left>a[i].right)
                swap(a[i].left,a[i].right);
        }
        sort(a,a+n,cmp);
        /*for(int i=0;i=min)
                       max=max>a[i].right?max:a[i].right;
                else if(a[i].left>min)
                {
                    top=i;
                    break;
                }
            if(min>max)
               break;
            else
               sum++;
        }
        if(max>=c)
         cout << sum <