poj 2431 Expedition （貪心+優先隊列）

poj 2431 Expedition （貪心+優先隊列）

Expedition Time Limit: 1000MS Memory Limit: 65536K Total Submissions: 6890 Accepted: 2065

Description

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.

Source

USACO 2005 U S Open Gold 比較簡單的一個題目，只不過題目意思比較難以理解，我讀了好幾遍題目才把意思弄明白，題目意思是一輛卡車距離城鎮有L距離，車中還有P油量，每向前行駛一單位距離消耗一單位的油量，如果在途中卡車中就沒有油了 ，就無法到達終點；途中有N個加油站，加油站提供的油量有限，卡車的油箱是無限的，給出每個加油站距離終點的距離和提供的油量，問卡車在途中最少需要加幾次油。 這個題目就要用到貪心的思想，先去提供油量比較多的地方，距離又比較近的地方，用優先隊列來儲存每一次的油量；只要油量恰好能夠到達終點，所加的次數就是最小的；

#include 
#include 
#include 
#include 
using namespace std;
const int maxn=10001;
typedef struct node
{
    int dis,fuel;
}node;
node stop[maxn];
bool cmp(node x,node y)//比較大小，題目中給的是到終點的距離
{
   return x.dis>y.dis;
}
int main()
{
    int n,l,p;
    int ans=0,j=0,temp;
    while(scanf("%d",&n)!=EOF)
    {
    priority_queueq;
    for(int i=0;i0 && !q.empty())
    {
        ans++;
        temp=q.top();
        q.pop();
        l-=temp;//油量能夠支撐的距離
        while(j