diff --git a/project_euler/problem_038/__init__.py b/project_euler/problem_038/__init__.py new file mode 100644 index 000000000000..e69de29bb2d1 diff --git a/project_euler/problem_038/sol1.py b/project_euler/problem_038/sol1.py new file mode 100644 index 000000000000..6d54f6df7ff8 --- /dev/null +++ b/project_euler/problem_038/sol1.py @@ -0,0 +1,77 @@ +""" +Project Euler Problem 38: https://projecteuler.net/problem=38 + +Take the number 192 and multiply it by each of 1, 2, and 3: + +192 × 1 = 192 +192 × 2 = 384 +192 × 3 = 576 + +By concatenating each product we get the 1 to 9 pandigital, 192384576. We will call +192384576 the concatenated product of 192 and (1,2,3) + +The same can be achieved by starting with 9 and multiplying by 1, 2, 3, 4, and 5, +giving the pandigital, 918273645, which is the concatenated product of 9 and +(1,2,3,4,5). + +What is the largest 1 to 9 pandigital 9-digit number that can be formed as the +concatenated product of an integer with (1,2, ... , n) where n > 1? + +Solution: +Since n>1, the largest candidate for the solution will be a concactenation of +a 4-digit number and its double, a 5-digit number. +Let a be the 4-digit number. +a has 4 digits => 1000 <= a < 10000 +2a has 5 digits => 10000 <= 2a < 100000 +=> 5000 <= a < 10000 + +The concatenation of a with 2a = a * 10^5 + 2a +so our candidate for a given a is 100002 * a. +We iterate through the search space 5000 <= a < 10000 in reverse order, +calculating the candidates for each a and checking if they are 1-9 pandigital. + +In case there are no 4-digit numbers that satisfy this property, we check +the 3-digit numbers with a similar formula (the example a=192 gives a lower +bound on the length of a): +a has 3 digits, etc... +=> 100 <= a < 334, candidate = a * 10^6 + 2a * 10^3 + 3a + = 1002003 * a +""" + +from typing import Union + + +def is_9_pandigital(n: int) -> bool: + """ + Checks whether n is a 9-digit 1 to 9 pandigital number. + >>> is_9_pandigital(12345) + False + >>> is_9_pandigital(156284973) + True + >>> is_9_pandigital(1562849733) + False + """ + s = str(n) + return len(s) == 9 and set(s) == set("123456789") + + +def solution() -> Union[int, None]: + """ + Return the largest 1 to 9 pandigital 9-digital number that can be formed as the + concatenated product of an integer with (1,2,...,n) where n > 1. + """ + for base_num in range(9999, 4999, -1): + candidate = 100002 * base_num + if is_9_pandigital(candidate): + return candidate + + for base_num in range(333, 99, -1): + candidate = 1002003 * base_num + if is_9_pandigital(candidate): + return candidate + + return None + + +if __name__ == "__main__": + print(f"{solution() = }")