USACO 1.3.1 Mixing Milk

本题利用贪心算法即可求解。贪心策略：优先取单价最低的，直到数量足够。

代码如下：

/*
ID: michael139
LANG: C
PROG: milk
*/
#include<stdio.h>
#include<string.h>
typedef struct {
    int p,a;
} Farmer;
Farmer f[5050],temp;
int main () {
    FILE *fin  = fopen("milk.in", "r");
    FILE *fout = fopen("milk.out", "w");
    int n,m,i,j,ans;
    while (fscanf(fin,"%d%d",&n,&m) != EOF) {
        for (i=1;i<=m;i++) fscanf(fin,"%d%d",&f[i].p,&f[i].a);
        for (i=1;i<=m;i++) {
            for (j=i+1;j<=m;j++) {
                if (f[i].p>f[j].p) {
                    temp = f[i];
                    f[i] = f[j];
                    f[j] = temp;
                }
            }
        }
        ans = 0;
        for (i=1;i<=m;i++) {
            if (f[i].a<=n) {
                ans += f[i].a*f[i].p;
                n -= f[i].a;
                if (n==0) break;
            } else {
                ans += n*f[i].p;
                break;
            }
        }
        fprintf(fout,"%d\n",ans);
    }
    return 0;
}