Find the Access Codes
Write a function answer(l) that takes a list of positive integers l and counts the number of “lucky triples” of (lst[i], lst[j], lst[k]) where i < j < k. The length of l is between 2 and 2000 inclusive. The elements of l are between 1 and 999999 inclusive. The answer fits within a signed 32-bit integer. Some of the lists are purposely generated without any access codes to throw off spies, so if no triples are found, return 0.
For example, [1, 2, 3, 4, 5, 6] has the triples: [1, 2, 4], [1, 2, 6], [1, 3, 6], making the answer 3 total.
Test cases
Inputs: (int list) l = [1, 1, 1] Output: (int) 1
Inputs: (int list) l = [1, 2, 3, 4, 5, 6] Output: (int) 3
My Attempt
from itertools import combinations def answer(l): if len(l) < 3: return 0 found = 0 for val in combinations(l,3): # Ordering Check if (val[0] <= val[1] <= val[2]) != True: continue # Answer Size Check against size of signed integer 32 bit if int(val[0].__str__() + val[1].__str__() + val[2].__str__()) > 2147483647: continue # Division Check if (val[1] % val[1] != 0) or (val[2] % val[1] != 0): continue # Increment 'found' variable by one found += 1 return found
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Answer
Here is a solution off the top of my head that has O(n^2) time and O(n) space complexity. I think there is a better solution (probably using dynamic programming), but this one beats generating all combinations.
public static int foobar( int[] arr) { int noOfCombinations = 0; int[] noOfDoubles = new int[arr.length]; // Count lucky doubles for each item in the array, except the first and last items for( int i = 1; i < arr.length-1; ++i) { for( int j = 0; j < i; ++j) { if( arr[i] % arr[j] == 0) ++noOfDoubles[i]; } } // Count lucky triples for( int i = 2; i < arr.length; i++) { for( int j = 1; j < i; ++j) { if( arr[i] % arr[j] == 0) noOfCombinations += noOfDoubles[j]; } } return noOfCombinations; }