diff --git a/project_euler/problem_061/__init__.py b/project_euler/problem_061/__init__.py new file mode 100644 index 000000000000..e69de29bb2d1 diff --git a/project_euler/problem_061/sol1.py b/project_euler/problem_061/sol1.py new file mode 100644 index 000000000000..c4146f4f0364 --- /dev/null +++ b/project_euler/problem_061/sol1.py @@ -0,0 +1,133 @@ +""" +Project Euler Problem 61: https://projecteuler.net/problem=61 + +Triangle, square, pentagonal, hexagonal, heptagonal, and octagonal numbers are a +ll figurate (polygonal) numbers and are generated by the following formulae: + +Triangle P(3,n)=n(n+1)/2 1, 3, 6, 10, 15, ... +Square P(4,n)=n^2 1, 4, 9, 16, 25, ... +Pentagonal P(5,n)=n(3n-1)/2 1, 5, 12, 22, 35, ... +Hexagonal P(6,n)=n(2n-1) 1, 6, 15, 28, 45, ... +Heptagonal P(7,n)=n(5n-3)/2 1, 7, 18, 34, 55, ... +Octagonal P(8,n)=n(3n-2) 1, 8, 21, 40, 65, ... + +The ordered set of three 4-digit numbers: 8128, 2882, 8281, has +three interesting properties. + + 1. The set is cyclic, in that the last two digits of each number is the first two + digits of the next number (including the last number with the first). + 2. Each polygonal type: triangle (P(3,127) = 8128), square (P(4,91) = 8281), and + pentagonal (P(5,44) = 2882),is represented by a different number in the set. + 3. This is the only set of 4-digit numbers with this property. + +Find the sum of the only ordered set of six cyclic 4-digit numbers for which each +polygonal type: triangle, square, pentagonal, hexagonal, heptagonal, and octagonal, +is represented by a different number in the set. +""" + +from itertools import permutations + + +def solution() -> int: + """ + For this task is good to know some basics of combinatorial analysis, and + know how to solve a quadratic equation. + + On the first sight we can see that number of possibilities is quite big. But + from the description of the task, we can get to know that: + 1. We are only looking for 4 digits numbers + 2. All numbers are naturals numbers. + + By knowing that we can try to limit range of numbers that + we will be looking for specific formulae. + + We need to solve some quadratic equation to get that range: + 1. Triangle: + Lower: n(n+1)/2 = 1000 // 45 + + ###### + We get two solutions one positive, one negative. From previous observation + we take positive one, and round it down or up, so + we get maximum 4 digit result. We doing that for rest of formulae. + ###### + + Upper: n(n+1)/2 = 10000 // 140 + + 2. Square: + Lower: n^2 = 1000 // 32 + Upper: n^2 = 9999 // 99 + 3. Pentagonal: + Lower: n(3n-1)/2 = 1000 // 26 + Upper: n(3n-1)/2 = 9999 // 81 + 4. Hexagonal: + Lower: n(2n-1) = 1000 // 23 + Upper: n(2n-1) = 9999 // 70 + 5. Heptagonal: + Lower: n(5n-3)/2 = 1000 // 21 + Upper: n(5n-3)/2 = 9999 // 63 + 6. Octagonal + Lower: n(3n-2) = 1000 // 19 + Upper: n(3n-2) = 9999 // 59 + + Then we just need to check if the last two digits of each number + is the first two digits of the next number, and if they are not the same. + + I created a function is_cyclic to check if it the last two digits of number is + the first two digits of the next number. + + Then program iterates through all permutations of polygonal types. (itertools). + For each permutation, it iterates through the corresponding lists of + polygonal numbers stored in the polygonals dictionary. + + At the end if 6 polygonal numbers form a cyclic set, it returns the sum of them. + """ + + triangle = [int(x * (x + 1) * 0.5) for x in range(45, 141)] + + square = [int(x * x) for x in range(32, 100)] + + pentagonal = [int(x * (3 * x - 1) * 0.5) for x in range(26, 82)] + + hexagonal = [int(x * (2 * x - 1)) for x in range(23, 71)] + + heptagonal = [int(x * (5 * x - 3) * 0.5) for x in range(21, 64)] + + octagonal = [int(x * (3 * x - 2)) for x in range(19, 60)] + + polygonals = { + 3: triangle, + 4: square, + 5: pentagonal, + 6: hexagonal, + 7: heptagonal, + 8: octagonal, + } + + for perm in permutations(range(3, 9)): + for t in polygonals[perm[0]]: + for s in polygonals[perm[1]]: + if not is_cyclic(t, s): + continue + for p in polygonals[perm[2]]: + if not is_cyclic(s, p): + continue + for hx in polygonals[perm[3]]: + if not is_cyclic(p, hx): + continue + for hp in polygonals[perm[4]]: + if not is_cyclic(hx, hp): + continue + for o in polygonals[perm[5]]: + if not is_cyclic(hp, o) or not is_cyclic(o, t): + continue + return sum([t, s, p, hx, hp, o]) + + return 0 + + +def is_cyclic(a: int, b: int): + return str(a)[2:] == str(b)[:2] + + +if __name__ == "__main__": + print(f"{solution() = }")