P2878 [USACO07JAN]Protecting the Flowers S

难度：3

主要就是分析贪心的策略，这个好像就是用到了邻项分析的方法，设两个例子，相邻的，然后看哪个排前面消耗少，哪个就排前面，排序的依据就是这里来的，

#include <bits/stdc++.h>

#define fi first
#define se second
#define pb push_back
#define all(x) (x).begin(), (x).end()

using namespace std;

typedef long long ll;
typedef vector<int> vi;
typedef pair<int, int> pa;

const int N = 100005;

struct node {
    int t, d;
    node(int a = 0, int b = 0): t(a), d(b) {}
} rec[N];

bool cmp(node a, node b) {
    return a.t * b.d < b.t * a.d;
}

int main() {
    int n;
    cin >> n;
    for (int i = 0; i < n; i++) {
        int t, d;
        cin >> t >> d;
        rec[i] = node(t, d);
    }
    sort(rec, rec + n, cmp);
    int minute = 0;
    ll ans = 0;
    for (int i = 0; i < n; i++) {
        ans += minute * rec[i].d;
        minute += rec[i].t * 2;
    }
    cout << ans;
    return 0;
}