Think about stable sort in ruby(1.9.3)

gistfile1.txt

[10] pry(main)> [[10, 120], [20, 190], [10, 110], [20, 220], [10, 130], [20, 180]].sort_by(&:first)
=> [[10, 120], [10, 130], [10, 110], [20, 220], [20, 190], [20, 180]]

[12] pry(main)> [[10, 120], [20, 190], [10, 110], [20, 220], [10, 130], [20, 180]].each.with_index.sort_by { |a, i| [a.first, i] }
=> [[[10, 120], 0],
 [[10, 110], 2],
 [[10, 130], 4],
 [[20, 190], 1],
 [[20, 220], 3],
 [[20, 180], 5]]

[13] pry(main)> [[10, 120], [20, 190], [10, 110], [20, 220], [10, 130], [20, 180]].each.with_index.sort_by { |a, i| [a.first, i] }.map(&:first)
=> [[10, 120], [10, 110], [10, 130], [20, 190], [20, 220], [20, 180]]