1152 Google Recruitment

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题目

In July 2004, Google posted on a giant billboard along Highway 101 in Silicon Valley (shown in the picture below) for recruitment. The content is super-simple, a URL consisting of the first 10-digit prime found in consecutive digits of the natural constant e. The person who could find this prime number could go to the next step in Google's hiring process by visiting this website.

prime.jpg

The natural constant e is a well known transcendental number（超越数）. The first several digits are: e = 2.718281828459045235360287471352662497757247093699959574966967627724076630353547594571382178525166427427466391932003059921... where the 10 digits in bold are the answer to Google's question.

Now you are asked to solve a more general problem: find the first K-digit prime in consecutive digits of any given L-digit number.

Input Specification:

Each input file contains one test case. Each case first gives in a line two positive integers: L (≤ 1,000) and K (< 10), which are the numbers of digits of the given number and the prime to be found, respectively. Then the L-digit number N is given in the next line.

Output Specification:

For each test case, print in a line the first K-digit prime in consecutive digits of N. If such a number does not exist, output 404 instead. Note: the leading zeroes must also be counted as part of the K digits. For example, to find the 4-digit prime in 200236, 0023 is a solution. However the first digit 2 must not be treated as a solution 0002 since the leading zeroes are not in the original number.

Sample Input 1:

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2
20 5
23654987725541023819

Sample Output 1:

1
49877

Sample Input 2:

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2
10 3
2468024680

Sample Output 2:

1
404

题意

找出给定长度的正整数L内的第一个长度为K的素数.

  • 正整数内的0也算作长度,例如200236内找出第一个长度为4的素数,那0023就是答案.
  • 如果没有要求的素数,输出404.

思路

这道题还是相对简单的,因为主要的考点还是在与判断某个数是不是素数,而判断素数的函数已经写过很多次了.

另一方面要获得指定长度的字符串,只需要用字符串的Substring()方法就可以了.

代码

先实现一个判断是否为素数的函数:

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public static bool IsPrime(int input)
{
    if (input <= 1)
    {
        return false;
    }
    for (int i = 2; i <= Math.Sqrt(input); i++)
    {
        if (input % i == 0)
        {
            return false;
        }
    }
    return true;
}
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public static void Main()
{
    int primeLength = int.Parse(Console.ReadLine().Split()[1]);
    string input = Console.ReadLine();
    for (int i = 0; i <= input.Length - primeLength; i++)
    {
        if (IsPrime(int.Parse(input.Substring(i, primeLength))))
        {
            Console.WriteLine(input.Substring(i, primeLength));
            return;
        }
    }
    Console.WriteLine("404");
}