Descriptively, scan through the slots in order, stacking reds up on the left, and blues up on the right, leaving greens in the middle. You only need to remembe

Descriptively, scan through the slots in order, stacking reds up on the left, and blues up on the right, leaving greens in the middle. You only need to remember how many reds & blues you've stacked up. Programmatically: LastRed = -1, LastBlue = numSlots - If the observed ball is red, swap with ++LastRed. Move the observing arm forward, as you've already observed the one swapped in. - If the observed ball is blue, swap with --LastBlue. - If the observed ball is green, leave it there. It might get swapped with a red later as LastRed grows. Move the observing arm forward. If the observing arm is at LastBlue, we're done. Loop.