[usaco]Mixing Milk题解_mixing milk(java)-优快云博客

本文链接：https://blog.youkuaiyun.com/xcwhkh/article/details/72599613

本文介绍了一个经典的贪心算法案例——购买牛奶问题。通过分析样例输入和输出，详细解释了如何通过排序和逐个选取最低价格的牛奶来实现最小化成本的目标。

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题目：

输入n和m表示需要n数量的牛奶，有m个商人提供。然后输入m组，每组两个数，分别代表单价和能提供的数量。

样例：

SAMPLE INPUT (file milk.in)

INPUT EXPLANATION

100 5 -- MMM wants 100 units of milk from 5 farmers
5 20 -- Farmer 1 says, "I can sell you 20 units at 5 cents per unit"
9 40 etc.
3 10 -- Farmer 3 says, "I can sell you 10 units at 3 cents per unit"
8 80 etc.
6 30 -- Farmer 5 says, "I can sell you 30 units at 6 cents per unit"

OUTPUT FORMAT

A single line with a single integer that is the minimum cost that Merry Milk Makers must pay for one day's milk.

SAMPLE OUTPUT (file milk.out)

OUTPUT EXPLANATION

Here's how the MMM company spent only 630 cents to purchase 100 units of milk:

Price per unit	Units available	Units bought	Price * # units	Total cost	Notes
5	20	20	5*20	100
9	40	0			Bought no milk from farmer 2
3	10	10	3*10	30
8	80	40	8*40	320	Did not buy all 80 units!
6	30	30	6*30	180
Total	180	100		630	Cheapest total cost

题解：

他告诉我是贪心算法了（虽然没学过）。就是单价排个序，然后从最小的拿起，能拿光就拿光，拿满就好了，思路可以说是非常好想。单价排序我就用了sort，然后为了绑定单价和数量，我只能想到用一个结构体，因此就必须在sort函数的第三个参数中自定义一个Bool型的函数，从而使你能够使用结构体数组作为参数进行排序。期间学了一手sort的更多用法。

代码：

/*
ID: xcwhkh1
PROG: milk
LANG: C++
*/
#include <iostream>
#include <fstream>
#include <string>
#include <algorithm>
using namespace std;
struct milk{
	int price,amt;
};
bool cp(milk m1,milk m2)
{
	return m1.price<m2.price;
}
int main() {
    ofstream fout ("milk.out");
    ifstream fin ("milk.in");
    int n,m,i,sum=0;//n是总数，m是人数 
	fin>>n>>m;
	struct milk mk[5001];
	for(i=0;i<m;i++)
		fin>>mk[i].price>>mk[i].amt;
	sort(mk,&mk[i],cp);
	i=0;
	while(mk[i].amt<n)
	{
		n-=mk[i].amt;
		sum+=mk[i].amt*mk[i].price;
		i++;
	}
	sum+=n*mk[i].price;
	fout<<sum<<endl;
    return 0;
}