Distributing Ballot Boxes

Today, besides SWERC'11, another important event is taking place in Spain which rivals it in importance: General Elections. Every single resident of the country aged 18 or over is asked to vote in order to choose representatives for the Congress of Deputies and the Senate. You do not need to worry that all judges will suddenly run away from their supervising duties, as voting is not compulsory. 
 The administration has a number of ballot boxes, those used in past elections. Unfortunately, the person in charge of the distribution of boxes among cities was dismissed a few months ago due to nancial restraints. As a consequence, the assignment of boxes to cities and the lists of people that must vote in each of them is arguably not the best. Your task is to show how efficiently this task could have been done. 
 The only rule in the assignment of ballot boxes to cities is that every city must be assigned at least one box. Each person must vote in the box to which he/she has been previously assigned. Your goal is to obtain a distribution which minimizes the maximum number of people assigned to vote in one box. 
 In the first case of the sample input, two boxes go to the fi rst city and the rest to the second, and exactly 100,000 people are assigned to vote in each of the (huge!) boxes in the most efficient distribution. In the second case, 1,2,2 and 1 ballot boxes are assigned to the cities and 1,700 people from the third city will be called to vote in each of the two boxes of their village, making these boxes the most crowded of all in the optimal assignment.

Input

The fi rst line of each test case contains the integers N (1<=N<=500,000), the number of cities, and B(N<=B<=2,000,000), the number of ballot boxes. Each of the following N lines contains an integer a i,(1<=a i<=5,000,000), indicating the population of the i th city. 
 A single blank line will be included after each case. The last line of the input will contain -1 -1 and should not be processed.

Output

For each case, your program should output a single integer, the maximum number of people assigned to one box in the most efficient assignment.

Sample Input

2 7
200000
500000

4 6
120
2680
3400
200

-1 -1

Sample Output

100000
1700

题意：有n个城市b个信箱，下面的n行是每个城市的人口数量，问怎样分配这些信箱才能使每个信箱可用的人数最多，并且保证每个城市必须要有信箱。

思路：这道题用二分法，先用打擂台法找出城市的最大人口数置为maxx，然后让minn=0；num为二者之和的一半，然后for每个城市的人口数除以num得到那个城市需要的信箱数量。然后加起来如果最终的信箱数量过大，证明需要向右走了，反之向左走。知道while循环结束，输出maxx即可。代码如下：

#include<stdio.h>  //二分法
#include<math.h>
int s[550000];
int main()
{
    int a,b,sum,minn,maxx,num,flag;
    while(~scanf("%d %d",&a,&b)&&(a!=-1||b!=-1))
    {
        int i;
        minn=0;
        for(i=0;i<a;i++)
        {
            scanf("%d",&s[i]);
            if(s[i]>maxx)
                maxx=s[i];  //找出最大的
        }
        while(minn<maxx)
        {
            num=(maxx+minn)/2;  //取中间
            sum=0;              //sum是计算需要多少个信箱
            flag=0;            //标记是不是maxx太小
            for(i=0;i<a;i++)
            {
                sum=sum+ceil((s[i]+0.0)/num);  //ceil用法：取不小于给定实数的最小整数
                if(sum>b)  //maxx太小了
                {
                    flag=1;
                }
            }
            if(flag==1)
                minn=num+1;  //往右走
            else
                maxx=num;    //往左走
        }
        printf("%d\n",maxx);
    }
    return 0;
}