[USACO 1.3.1]混合牛奶 (贪心法)

题目：http://hustoj.sinaapp.com/problem.php?id=1825

制造商从多个原材料提供商获取原材料，使费用最小

思路：快排，贪心法

#include <stdio.h>
#include <stdlib.h>

struct milksuply
{
    int cost;
    int num;
}suply[5005];

int cmp(const void *a,const void *b)
{
    return ((milksuply *)a)->cost-((milksuply *)b)->cost;
}

int main()
{
    int n,m;
    scanf("%d%d",&n,&m);
    for(int i=0;i<m;i++)
    {
        scanf("%d%d",&suply[i].cost,&suply[i].num);
    }
    qsort(suply,m,sizeof(milksuply),cmp);

    int sum=0;
    int ans=0;
    for(int i=0;i<m;i++)
    {
        if(sum+suply[i].num>n)
        {
            ans+=(n-sum)*suply[i].cost;
            break;
        }
        else
        {
            sum+=suply[i].num;
            ans+=suply[i].num*suply[i].cost;
        }
    }
    printf("%d\n",ans);
}