HDU-1009FatMouse' Trade

FatMouse' Trade

 

Time Limit: 2000/1000 MS (Java/Others)    Memory Limit: 65536/32768 K (Java/Others)

 

题目链接

http://acm.hdu.edu.cn/showproblem.php?pid=1009

 

Problem Description

FatMouse prepared M pounds of cat food, ready to trade with the cats guarding the warehouse containing his favorite food, JavaBean. 
The warehouse has N rooms. The i-th room contains J[i] pounds of JavaBeans and requires F[i] pounds of cat food. FatMouse does not have to trade for all the JavaBeans in the room, instead, he may get J[i]* a% pounds of JavaBeans if he pays F[i]* a% pounds of cat food. Here a is a real number. Now he is assigning this homework to you: tell him the maximum amount of JavaBeans he can obtain. 

 

 

Input

The input consists of multiple test cases. Each test case begins with a line containing two non-negative integers M and N. Then N lines follow, each contains two non-negative integers J[i] and F[i] respectively. The last test case is followed by two -1's. All integers are not greater than 1000. 

 

 

Output

For each test case, print in a single line a real number accurate up to 3 decimal places, which is the maximum amount of JavaBeans that FatMouse can obtain. 

 

 

Sample Input

5 3

7 2

4 3

5 2

20 3

25 18

24 15

15 10

-1 -1

 

 

Sample Output

13.333

31.50

 

总结

 

初步了解了贪心算法，是选择价值比最大的东西来拿（由于可以切开拿，不是必须整体），接下来就好多了。

遇到的问题主要是浮点数的误差，只能用价值比（浮点数）比较大小，后面的运算求和时不能用它相乘来计算，拿样例来说，结果是13.333，而我没改进前，算出来是12.444.可见是误差。

第二个就是网上其他人使用结构体来储存，这样排序时其他变量能一起移动，但我开了3个数组，而且也不能用sort来排序了，只能自己写了选排。

#include<iostream>
#include<cstdio>
using namespace std;
double price[1005];
int num1[1005],num2[1005];

void sort1(double* a,int x)
{
    int b;
    double c;
    for(int i=0;i<x;i++){
        for(int j=i+1;j<x;j++){
            if(a[i]<a[j]){
                c=a[i];
                a[i]=a[j];
                a[j]=c;
                b=num1[i];
                num1[i]=num1[j];
                num1[j]=b;
                b=num2[i];
                num2[i]=num2[j];
                num2[j]=b;
            }
        }
    }
}

int main()
{
    int m,n,j;
    while(scanf("%d %d",&m,&n)==2){
        if(m==-1&&n==-1) break;
        for(int i=0;i<n;i++){
            scanf("%d %d",&num2[i],&num1[i]);
            price[i]=(double)num2[i]/num1[i];
        }
        sort1(price,n);
        //for(int i=0;i<n;i++) cout <<price[i]<<num[i]<<endl;
        double sum=0;
        for(int i=0;i<n;i++){
            if(m<=0) break;
            if(m>=num1[i]){
                sum+=num2[i];
                m-=num1[i];
            }
            else {
                sum+=(double)m/num1[i]*num2[i];
                m=0;
            }
        }
        printf("%.3lf\n",sum);
    }
    return 0;
}