Assume that we are given n balls as input, where the ball has a number and is one of three colors (red, blue, or yellow). Further assume that the balls are sorted by their number. Give an O(n) algorithm to sort the balls by color (all reds before all blues before all yellows) such that the numbers for identical colors stay sorted. For example: (1,blue), (3,red), (4,blue), (6,yellow), (9,red) should become (3,red), (9,red), (1,blue), (4,blue), (6,yellow).