Python/project_euler/problem_37/sol1.py
Wen Hong 6541236fdf
Add solution method for project_euler/problem_37 (#2998)
* add solution method

* fix formatting
2020-10-08 09:27:14 +05:30

99 lines
2.6 KiB
Python

"""
The number 3797 has an interesting property. Being prime itself, it is possible
to continuously remove digits from left to right, and remain prime at each stage:
3797, 797, 97, and 7. Similarly we can work from right to left: 3797, 379, 37, and 3.
Find the sum of the only eleven primes that are both truncatable from left to right
and right to left.
NOTE: 2, 3, 5, and 7 are not considered to be truncatable primes.
"""
from __future__ import annotations
seive = [True] * 1000001
seive[1] = False
i = 2
while i * i <= 1000000:
if seive[i]:
for j in range(i * i, 1000001, i):
seive[j] = False
i += 1
def is_prime(n: int) -> bool:
"""
Returns True if n is prime,
False otherwise, for 1 <= n <= 1000000
>>> is_prime(87)
False
>>> is_prime(1)
False
>>> is_prime(25363)
False
"""
return seive[n]
def list_truncated_nums(n: int) -> list[int]:
"""
Returns a list of all left and right truncated numbers of n
>>> list_truncated_nums(927628)
[927628, 27628, 92762, 7628, 9276, 628, 927, 28, 92, 8, 9]
>>> list_truncated_nums(467)
[467, 67, 46, 7, 4]
>>> list_truncated_nums(58)
[58, 8, 5]
"""
str_num = str(n)
list_nums = [n]
for i in range(1, len(str_num)):
list_nums.append(int(str_num[i:]))
list_nums.append(int(str_num[:-i]))
return list_nums
def validate(n: int) -> bool:
"""
To optimize the approach, we will rule out the numbers above 1000,
whose first or last three digits are not prime
>>> validate(74679)
False
>>> validate(235693)
False
>>> validate(3797)
True
"""
if len(str(n)) > 3:
if not is_prime(int(str(n)[-3:])) or not is_prime(int(str(n)[:3])):
return False
return True
def compute_truncated_primes(count: int = 11) -> list[int]:
"""
Returns the list of truncated primes
>>> compute_truncated_primes(11)
[23, 37, 53, 73, 313, 317, 373, 797, 3137, 3797, 739397]
"""
list_truncated_primes = []
num = 13
while len(list_truncated_primes) != count:
if validate(num):
list_nums = list_truncated_nums(num)
if all(is_prime(i) for i in list_nums):
list_truncated_primes.append(num)
num += 2
return list_truncated_primes
def solution() -> int:
"""
Returns the sum of truncated primes
"""
return sum(compute_truncated_primes(11))
if __name__ == "__main__":
print(f"{sum(compute_truncated_primes(11)) = }")