62_unique_paths.rb

def unique_paths(m, n)
  grid = Array.new(m)

  m.times.with_index do |y|
    grid[y] = Array.new(n)
    grid[y][0] = 1

    n.times.with_index do |x|
      from_left = if x - 1 >= 0 then grid[y][x - 1] else 0 end
      from_above = if y - 1 >= 0 then grid[y - 1][x] else 0 end

      grid[y][x] ||= from_left + from_above
    end
  end

  grid.last.last
end