POJ 2431 Expedition

贪心，及优先队列或堆的使用。当路过一个城市或加油站时，不一定需要立刻加油，而可以将其保存在一个优先队列中。当没有油时，再取出优先队列的头加入等量的油，加油的先后是等价的。以加油站为分段点考虑，并将距离转化为距起点的距离。距离需要排序，输入并不保证有序，否则会wrong answer。

#include <iostream>#include <cstdio>#include <queue>#include <algorithm>using namespace std;int n;struct City{int a, b;}city[10010];bool cmp(const City& x, const City& y){return x.a < y.a;}int l, p;void solve()  //  use the priority queue{int ans = 0, tank = p, pos = 0;priority_queue<int> q;for (int i = 0; i <= n; i++){int d = city[i].a - pos;  //  distance to next citywhile (tank < d)  //  need to go to the gas station{if (q.empty())  //  no gas station to use{printf("-1\n");  //  cannot go to the final cityreturn;}//  need to use the gas stationtank += q.top();ans++;q.pop();}//  already get to the next citytank -= d;pos = city[i].a;q.push(city[i].b);}printf("%d\n", ans);}int main(){scanf("%d", &n);for (int i = 0; i < n; i++)scanf("%d%d", &city[i].a, &city[i].b);scanf("%d%d", &l, &p);for (int i = 0; i < n; i++)city[i].a = l - city[i].a;city[n].a = l;city[n].b = 0;//  without sort will cause wrong answer.sort(city, city + n + 1, cmp);solve();}