【Project Euler】【Problem 3】Largest prime factor

Largest prime factor

Problem 3

The prime factors of 13195 are 5, 7, 13 and 29.

What is the largest prime factor of the number 600851475143 ?

Answer: 6857

最大质因子

Problem 3

13195的质因子是5,7,13,29

600851475143 的最大质因子是多少?

答案: 6857

解法一：

#include <stdio.h>
#include <math.h>

//判断质数
bool isPrime(long long num)
{
    if (num == 2 || num == 3)
    {
        return true;
    }

    long long _sqrt = (long long)sqrt(num);

    for (int i = 2; i <= _sqrt; i++ )
    {
        if (num % i == 0)
        {
            return false;
        }
    }
    return true;
}

int main(int argc, char *argv[])
{
    long long maxNum = 600851475143ll;
    long long quot = maxNum;
    long long _sqrt = (long long)sqrt(quot);

    while (!isPrime(quot))
    {
        for (int i = 2; i < _sqrt; i++)
        {
            if (quot % i ==0)
            {
                quot = quot / i;
				_sqrt = (long long)sqrt(quot);
                break;
            }
        }
    }

    printf("find Prime : %I64d \n",quot);

    return 0;
}