Expedition

题目： Expedition

poj-2431

题意:\
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.

首先，看到这题，给人的一个想法就是贪心。
问题就转化成了怎么贪的问题。
然后由于我们做到这题的，专题在于那个二叉树和堆的专题。
而且 我们也列出来了知识点的方向：优先队列，堆优化，Trie树；
额，所以我们很自然的想到了这个题的知识点在与
贪心和优先队列；
解释虽然很僵硬，但是事实就是这样。
题目好像还没有解释。
就是 一个人在路上，车坏了，或者说，油不够了，然后在回去的路上，有好几个加油站，每个加油站都能加一些油。然后问最少要经过几个加油站。
我写这题的目的就在于，纪念下，第一道优先队列的题。

#include<cstdio>#include<cstring>#include<iostream>#include<cmath>#include<algorithm>#include<queue>#include<vector>#define maxn 30005#define INF 0x3f3f3f3fusing namespace std;typedef long long ll;struct st{    int yuan;    int you;}node[10005];int cmp(st a, st b ){    if(a.yuan>b.yuan) return 1;    return 0;}int main(){    int n,k;    int l,p;   while(scanf("%d",&n)==1)   {      for(int i=0;i<n;i++)      {          scanf("%d%d",&node[i].yuan,&node[i].you);      }      sort(node,node+n,cmp);      scanf("%d%d",&l,&p);      priority_queue<int>q;      q.push(p);      int ans=0,i=0;      while(l>0 && !q.empty())      {          ans++;          l-=q.top();          q.pop();          while( i<n && l<=node[i].yuan)          q.push(node[i++].you);      }      printf("%d\n",l<=0?ans-1:-1);   }    return 0;}

用这题基础题，记录下自己的优先队列。该怎么写，stl还是很强的。