squares with three sides

approach

every line becomes a tuple of three integers. a triple is a valid triangle when each pair of sides adds up to more than the third, checked as three explicit comparisons.

part 1 counts the rows that pass. part 2 reads the triangles down the columns instead: the rows are cut into chunks of three, and for each column index the three values in the chunk are tested as one triangle.

the trick

part 2 doesn't re-parse anything: it takes three rows at a time and picks the same column out of each.

def chunks(arr, k):
    for i in range(0, len(arr), k):
        yield arr[i:i + k]

def part2(triangles, k):
    res = 0
    for i in range(k):
        for ch in chunks(triangles, k):
            res += are_triangle(*(c[i] for c in ch))
    return res