水题必须的，果断用struct排序再计算，第一次循环条件搞错WA了一次。。。

[cpp]
#include??
#include??
#define MAXN 10000??
using namespace std;?
?
struct Room{?
??? int j, f;?
??? double value;?
};?
Room r[MAXN];?
?
bool cmp(Room a, Room b)?
{?
??? return a.value > b.value;?
}?
?
int main()?
{?
??? int m, n;?
??? while (scanf("%d%d", &m, &n) && m != -1 && n != -1)?
??? {?
??????? for (int i = 0; i < n; i++)?
??????? {?
??????????? scanf("%d%d", &r[i].j, &r[i].f);?
??????????? r[i].value = (double) r[i].j / (double) r[i].f;?
??????? }?
??????? sort(r, r + n, cmp);??? //排序??
??????? double sum = 0;?
??????? for (int i = 0; i < n; i++)?
??????????? if (m >= r[i].f)?
??????????????? sum += r[i].j, m -= r[i].f; //循环加进能全部拿到的东西??
??????????? else?
??????????? {?
??????????????? sum += (double) m * r[i].value; //最后一个房间只能拿一部分??
??????????????? break;????????????????????????? //拿完跳出循环??
??????????? }?
??????? printf("%.3f\n", sum);?
??? }?
??? return 0;?
}?

#include
#include
#define MAXN 10000
using namespace std;

struct Room{
?int j, f;
?double value;
};
Room r[MAXN];

bool cmp(Room a, Room b)
{
?return a.value > b.value;
}

int main()
{
?int m, n;
?while (scanf("%d%d", &m, &n) && m != -1 && n != -1)
?{
??for (int i = 0; i < n; i++)
??{
???scanf("%d%d", &r[i].j, &r[i].f);
???r[i].value = (double) r[i].j / (double) r[i].f;
??}
??sort(r, r + n, cmp);?//排序
??double sum = 0;
??for (int i = 0; i < n; i++)
???if (m >= r[i].f)
????sum += r[i].j, m -= r[i].f;?//循环加进能全部拿到的东西
???else
???{
????sum += (double) m * r[i].value;?//最后一个房间只能拿一部分
????break;???????//拿完跳出循环
???}
??printf("%.3f\n", sum);
?}
?return 0;
}
慢慢地追随E.star大神的脚步了。。。