mtreinik / advent-of-code-2020

My participation to Advent of Code 2020

advent-of-code-2020

My participation to Advent of Code 2020

Day 1: Report Repair

Algorithm for part one

1. put input entries in an array numbers
2. sort array by ascending order
3. start first at first element of array
4. start second at last element of array
5. calculate sum numbers[first] + numbers[second]
6. if sum is less than 2020, move first to next element
7. if sum is greater than 2020, move second to previous element
8. if sum equals 2020, return numbers[first] * numbers[second]
9. if first and second point to the same element, no match was found
10. repeat from calculating the sum

Finding the matching pair of numbers has complexity O(n), which is dominated by complexity of sorting, which is, depending on implementation O(n log n).

Algorithm for part two

1. Run algorithm of part 1 for each number in the array, looking for sum that equals 2020 - number

This algorithm has complexity O(n^2).

Running the program

$ nvm use
$ ./day1.js < simple-input-day1.txt
part 1: 514579 part 2: 241861950

or

$ ./day1.awk < simple-input-day1.txt
part 1: 514579 part 2: 241861950