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[USACO 2007 Jan S]Protecting the Flowers

链接：ac.nowcoder.com/acm/problem…
来源：牛客网

题目描述

Farmer John went to cut some wood and left N (2 ≤ N ≤ 100,000) cows eating the grass, as usual. When he returned, he found to his horror that the cluster of cows was in his garden eating his beautiful flowers. Wanting to minimize the subsequent damage, FJ decided to take immediate action and transport each cow back to its own barn.
Each cow i is at a location that is Ti minutes (1 ≤ Ti ≤ 2,000,000) away from its own barn. Furthermore, while waiting for transport, she destroys Di (1 ≤ Di ≤ 100) flowers per minute. No matter how hard he tries, FJ can only transport one cow at a time back to her barn. Moving cow i to its barn requires 2 × Ti minutes (Ti to get there and Ti to return). FJ starts at the flower patch, transports the cow to its barn, and then walks back to the flowers, taking no extra time to get to the next cow that needs transport.
Write a program to determine the order in which FJ should pick up the cows so that the total number of flowers destroyed is minimized.

输入描述:

Line 1: A single integer N
Lines 2..N+1: Each line contains two space-separated integers, Ti and Di, that describe a single cow's characteristics

输出描述:

Line 1: A single integer that is the minimum number of destroyed flowers

示例1

输入

6
3 1
2 5
2 3
3 2
4 1
1 6

输出

86

思路

这个题读完会发现是一道有点巧妙地贪心问题，首先用结构体来存储需要的一些数据，然后用重写的cmp比较函数来让数组按照要求排列，再利用前缀和，因为送前面的牛离开后，后面的牛不会停止吃草，所以就把代码写出来啦。qwq

代码

#include<bits/stdc++.h>
using namespace std;
struct cow{
    int t;
    int d;
}a[100005];
bool cmp(cow a,cow b){
    return a.t*b.d<b.t*a.d;
}
int main()
{
    int n;
    cin>>n;
    for(int i=0;i<n;i++){
        cin>>a[i].t>>a[i].d;
    }
    sort(a,a+n,cmp);
    int tim=0;
    long long ans=0;
    for(int i=1;i<n;i++){
        ans+=(tim+a[i-1].t)*2*a[i].d;
        tim+=a[i-1].t;
    }
    cout<<ans;
    return 0;
}