Diagonal Difference - Hackerrank

n = gets.strip.to_i
a = Array.new(n)
for a_i in (0..n-1)
    a_t = gets.strip
    a[a_i] = a_t.split(' ').map(&:to_i)
    arr = a[a_i]
end


a_diagonal = (0...arr.length).collect { |h|
        a[h][h]
    }.reduce(:+)

rev_a = a.each { |i|
        i.reverse!
    }

b_diagonal = (0...arr.length).collect { |i|
        rev_a[i][i]
    }.reduce(:+)

(a_diagonal - b_diagonal).abs