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hdu 1018 Big Number

2024-07-16 21:45:40   227人阅读

Problem Description
In many applications very large integers numbers are required. Some of these applications are using keys for secure transmission of data, encryption, etc. In this problem you are given a number, you have to determine the number of digits in the factorial of the number.
 
Input
Input consists of several lines of integer numbers. The first line contains an integer n, which is the number of cases to be tested, followed by n lines, one integer 1 ≤ n ≤ 107 on each line.
 
Output
The output contains the number of digits in the factorial of the integers appearing in the input.
 
Sample Input
2
10
20
 
Sample Output
7
19
 
 
    这题是要求n的阶乘的位数，而n的阶乘是n个数的乘积，那么要是我们能把这个问题分解就好了。
  对于任意一个给定的正整数a，
  假设10^(x-1)<=a<10^x，那么显然a的位数为x位，
  又因为
  log10(10^(x-1))<=log10(a)<(log10(10^x))
  即x-1<=log10(a)<x
  则(int)log10(a)=x-1,
  即(int)log10(a)+1=x
  即a的位数是(int)log10(a)+1

我们知道了一个正整数a的位数等于(int)log10(a) + 1，
现在来求n的阶乘的位数：
假设A=n!=1*2*3*......*n，那么我们要求的就是
(int)log10(A)+1，而：
log10(A)=log10(1*2*3*......n)  （根据log10(a*b) = log10(a) + log10(b)有）
         =log10(1)+log10(2)+log10(3)+......+log10(n)
现在我们终于找到方法，问题解决了，我们将求n的阶乘的位
数分解成了求n个数对10取对数的和，并且对于其中任意一个数，
都在正常的数字范围之类。

总结一下：n的阶乘的位数等于
  (int)(log10(1)+log10(2)+log10(3)+......+log10(n)) + 1

根据这个思路我们很容易写出程序
 
 
 
#include<stdio.h>#include<math.h>int main(){    int i,n,t,j;    double r;    scanf("%d",&n);    for(i=0;i<n;i++)    {        r=0;        scanf("%d",&t);        for(j=1;j<=t;j++)        {            r+=log10(j);        }        printf("%d\n",(int)r+1);    }}

 


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