BZOJ 1620: [Usaco2008 Nov]Time Management 时间管理 (贪心)

1620: [Usaco2008 Nov]Time Management 时间管理

Time Limit: 5 Sec  Memory Limit: 64 MB Submit: 636  Solved: 387 [Submit][Status][Discuss]

Description

Ever the maturing businessman, Farmer John realizes that he must manage his time effectively. He has N jobs conveniently numbered 1..N (1 <= N <= 1,000) to accomplish (like milking the cows, cleaning the barn, mending the fences, and so on). To manage his time effectively, he has created a list of the jobs that must be finished. Job i requires a certain amount of time T_i (1 <= T_i <= 1,000) to complete and furthermore must be finished by time S_i (1 <= S_i <= 1,000,000). Farmer John starts his day at time t=0 and can only work on one job at a time until it is finished. Even a maturing businessman likes to sleep late; help Farmer John determine the latest he can start working and still finish all the jobs on time.

N个工作,每个工作其所需时间,及完成的Deadline,问要完成所有工作,最迟要什么时候开始.

Input

• Line 1: A single integer: N

• Lines 2..N+1: Line i+1 contains two space-separated integers: T_i and S_i

Output

• Line 1: The latest time Farmer John can start working or -1 if Farmer John cannot finish all the jobs on time.

Sample Input

4 3 5 8 14 5 20 1 16

INPUT DETAILS:

Farmer John has 4 jobs to do, which take 3, 8, 5, and 1 units of time, respectively, and must be completed by time 5, 14, 20, and 16, respectively.

Sample Output

2

OUTPUT DETAILS:

Farmer John must start the first job at time 2. Then he can do the second, fourth, and third jobs in that order to finish on time.

思路：贪心。一眼题。5分钟A掉，爽。 按照s[i]排序，然后完成前i个任务的最晚开始时间为 s[i]-sum ，sum为t[1]+...+t[i]..

如果ans<0,说明无法完成，赋值为-1.

/* ***********************************************
Author :111qqz
Created Time :2016年04月04日 星期一 01时36分31秒
File Name :code/bzoj/1620.cpp
************************************************ */

#include <cstdio>
#include <cstring>
#include <iostream>
#include <algorithm>
#include <vector>
#include <queue>
#include <set>
#include <map>
#include <string>
#include <cmath>
#include <cstdlib>
#include <ctime>
#define fst first
#define sec second
#define lson l,m,rt<<1
#define rson m+1,r,rt<<1|1
#define ms(a,x) memset(a,x,sizeof(a))
typedef long long LL;
#define pi pair < int ,int >
#define MP make_pair

using namespace std;
const double eps = 1E-8;
const int dx4[4]={1,0,0,-1};
const int dy4[4]={0,-1,1,0};
const int inf = 0x3f3f3f3f;
const int N=1E3+7;
pi a[N];
int n;
int main()
{
	#ifndef  ONLINE_JUDGE 
	freopen("code/in.txt","r",stdin);
  #endif
	cin>>n;
	for ( int i = 1 ; i <= n ; i++)
	{
	    int x,y;
	    cin>>x>>y;
	    a[i]=make_pair(y,x);
	}

	sort(a+1,a+n+1);
	int sum = 0 ;
	int ans = inf;
	for ( int i = 1 ; i <= n ; i++)
	{
	    sum += a[i].sec;
	    ans = min(ans,a[i].fst-sum);
	}
	if (ans<0) ans=-1;
	cout<<ans<<endl;

  #ifndef ONLINE_JUDGE  
  fclose(stdin);
  #endif
    return 0;
}