POJ 1064 Cable master(二分查找)

Cable master

Time Limit: 1000MS		Memory Limit: 10000K
Total Submissions: 31900		Accepted: 6827

Description
Inhabitants of the Wonderland have decided to hold a regional programming contest. The Judging Committee has volunteered and has promised to organize the most honest contest ever. It was decided
to connect computers for the contestants using a "star" topology - i.e. connect them all to a single central hub. To organize a truly honest contest, the Head of the Judging Committee has decreed to place all contestants evenly around the hub on an equal distance
from it. 

To buy network cables, the Judging Committee has contacted a local network solutions provider with a request to sell for them a specified number of cables with equal lengths. The Judging Committee wants the cables to be as long as possible to sit contestants
as far from each other as possible. 

The Cable Master of the company was assigned to the task. He knows the length of each cable in the stock up to a centimeter,and he can cut them with a centimeter precision being told the length of the pieces he must cut. However, this time, the length is not
known and the Cable Master is completely puzzled. 

You are to help the Cable Master, by writing a program that will determine the maximal possible length of a cable piece that can be cut from the cables in the stock, to get the specified number of pieces.
Input
The first line of the input file contains two integer numb ers N and K, separated by a space. N (1 = N = 10000) is the number of cables in the stock, and K (1 = K = 10000) is the number of
requested pieces. The first line is followed by N lines with one number per line, that specify the length of each cable in the stock in meters. All cables are at least 1 meter and at most 100 kilometers in length. All lengths in the input file are written
with a centimeter precision, with exactly two digits after a decimal point.
Output
Write to the output file the maximal length (in meters) of the pieces that Cable Master may cut from the cables in the stock to get the requested number of pieces. The number must be written
with a centimeter precision, with exactly two digits after a decimal point. 

If it is not possible to cut the requested number of pieces each one being at least one centimeter long, then the output file must contain the single number "0.00" (without quotes).
Sample Input

4 11
8.02
7.43
4.57
5.39

Sample Output

2.00

题意：有n段长度分别为Li的电缆，要求把它们分割成K条长度为X的电缆，问X的最大值为多少。

题解：将X视为变量，可知它的范围为0~max;  那么问题就变成了电缆长度取X时，所得的电缆条数大于，还是等于，或小于K的问题。  用二分查找法能很快的找出K的值，不过要注意精度，直接输出时会四舍五入，五入时的结果肯定是错的，注意向下取整。

代码如下：

#include<cstdio>
#include<cstring>
#include<cmath>
#define maxn 10010
#define INF 100001
double a[maxn];
int n,k;

bool dix(double x)
{
int num=0,i;
for(i=0;i<n;++i)
num+=(int)(a[i]/x);
return num>=k;//满足条件是返回true，不满足返回false
}

int main()
{
while(scanf("%d%d",&n,&k)!=EOF)
{
int i;
for(i=0;i<n;++i)
scanf("%lf",&a[i]);
double left=0,right=INF,mid;
i=1000;
while(i--)
{
mid=(left+right)/2;
if(dix(mid))
left=mid;
else
right=mid;
}
printf("%0.2f\n",floor(right*100)/100);//注意精度，可不明白%0.2lf输出为什么会错
}
return 0;
}