hoj1003 Mixing Milk (哈工大)

//贪心法，使用排序函数，每次都找最便宜的牛奶，然后判断够没够重量
#include <iostream>
#include <algorithm>
#define X 5010
using namespace std;

typedef struct milk        //定义牛奶结构体
{
    int p;
    int a;
}milk;

int cmp(milk a,milk b)        //对牛奶结构体排序
{
    return a.p<b.p;    
}

int main()
{
    int n,m;
    freopen("sum.in","r",stdin);
    freopen("sum.out","w",stdout);
    while(cin>>n>>m)
    {
        struct milk milk[X];
        int i;
        for(i=0;i<m;i++)
            cin>>milk[i].p>>milk[i].a;        //输入牛奶价格、重量
        sort(milk,milk+m,cmp);                //使用排序函数进行排序
        int sum = 0,amount = 0;
        for(i=0;i<m;i++)
        {
            if(amount+milk[i].a>=n)            //够了的话
            {
                int need = n-amount;
                sum = sum + need*milk[i].p;
                break;
            }
            else                            //牛奶还没够
            {
                sum =sum + milk[i].p*milk[i].a;
                amount+=milk[i].a;
            }
        }
        cout<<sum<<endl;
    }
    return 0;
}