H - Expedition 优先队列 贪心

Posted 2020-12-30 wzl19981116

来源poj2431

A group of cows grabbed a truck and ventured on an expedition deep into the jungle. Being rather poor drivers, the cows unfortunately managed to run over a rock and puncture the truck‘s fuel tank. The truck now leaks one unit of fuel every unit of distance it travels.

To repair the truck, the cows need to drive to the nearest town (no more than 1,000,000 units distant) down a long, winding road. On this road, between the town and the current location of the truck, there are N (1 <= N <= 10,000) fuel stops where the cows can stop to acquire additional fuel (1..100 units at each stop).

The jungle is a dangerous place for humans and is especially dangerous for cows. Therefore, the cows want to make the minimum possible number of stops for fuel on the way to the town. Fortunately, the capacity of the fuel tank on their truck is so large that there is effectively no limit to the amount of fuel it can hold. The truck is currently L units away from the town and has P units of fuel (1 <= P <= 1,000,000).

Determine the minimum number of stops needed to reach the town, or if the cows cannot reach the town at all.

Input

• Line 1: A single integer, N

• Lines 2..N+1: Each line contains two space-separated integers describing a fuel stop: The first integer is the distance from the town to the stop; the second is the amount of fuel available at that stop.

• Line N+2: Two space-separated integers, L and P

  Output

• Line 1: A single integer giving the minimum number of fuel stops necessary to reach the town. If it is not possible to reach the town, output -1.

  Sample Input

  4
  4 4
  5 2
  11 5
  15 10
  25 10

  Sample Output

  2

  Hint

  INPUT DETAILS:

The truck is 25 units away from the town; the truck has 10 units of fuel. Along the road, there are 4 fuel stops at distances 4, 5, 11, and 15 from the town (so these are initially at distances 21, 20, 14, and 10 from the truck). These fuel stops can supply up to 4, 2, 5, and 10 units of fuel, respectively.

OUTPUT DETAILS:

Drive 10 units, stop to acquire 10 more units of fuel, drive 4 more units, stop to acquire 5 more units of fuel, then drive to the town.

问你要到达目的地，最少需要下几次车,用贪心的思想每次只拿最大的，先对加油站的位置排个序，然后现在油量能到的加油站就push进优先队列，按油量大到小排，一次加一个，到了就跳出，队列空了就到不了

#include<iostream>
#include<stdio.h>
#include<stdlib.h>
#include <iomanip>
#include<cmath>
#include<float.h> 
#include<string.h>
#include<algorithm>
#define sf scanf
#define pf printf
#define scf(x) scanf("%d",&x)
#define scff(x,y) scanf("%d%d",&x,&y)
#define prf(x) printf("%d
",x) 
#define mm(x,b) memset((x),(b),sizeof(x))
#include<vector>
#include<queue>
#include<map>
#define rep(i,a,n) for (int i=a;i<n;i++)
#define per(i,a,n) for (int i=a;i>=n;i--)
typedef long long ll;
const ll mod=1e9+7;
const double eps=1e-8;
const int inf=0x3f3f3f3f;
using namespace std;
const double pi=acos(-1.0);
const int N=1e4+10;
struct node
{
    int pos,fue;
}a[N];
priority_queue<int>v;
bool cmp(node a,node b)
{
    return a.pos>b.pos;
}
int main()
{
    int ans=0,n,l,now;scf(n);
    rep(i,0,n)
    scff(a[i].pos,a[i].fue);
    scff(l,now);
    sort(a,a+n,cmp);
    int i=0;
    while(now<l)
    {
        for(;i<n;i++)
        {
            if(l-a[i].pos>now) break;
            v.push(a[i].fue); 
        }
        if(v.empty())
        {
            pf("-1
");return 0;
        }
        now+=v.top();
        v.pop();
        ans++;
    }
    prf(ans);
    return 0;
}