52 lines
1.3 KiB
Python
52 lines
1.3 KiB
Python
#!/usr/bin/python3
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# The prime factors of 13195 are 5, 7, 13 and 29.
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#
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# What is the largest prime factor of the number 600851475143?
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from math import floor, sqrt
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from timeit import default_timer
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from projecteuler import is_prime
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# Recursive approach: if num is prime, return num, otherwise
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# recursively look for the largest prime factor of num divided
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# by its prime factors until only the largest remains.
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def max_prime_factor(num):
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# Use function defined in projecteuler.py to check if a number is prime.
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if is_prime(num):
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return num
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# If num is even, find the largest prime factor of num/2.
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if num % 2 == 0:
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return max_prime_factor(num // 2)
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else:
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i = 3
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# If num is divisible by i and i is prime, find largest
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# prime factor of num/i.
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while True:
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if num % i == 0:
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if is_prime(i):
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return max_prime_factor(num//i)
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i = i + 2
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# Should never get here
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return -1
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def main():
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start = default_timer()
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res = max_prime_factor(600851475143)
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end = default_timer()
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print('Project Euler, Problem 3')
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print('Answer: {}'.format(res))
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print('Elapsed time: {:.9f} seconds'.format(end - start))
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if __name__ == '__main__':
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main()
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