Mauritus national flag problem

Question

I\'ve made a solution for the Dutch national flag problem already.

But this time, I want to try something more difficult: the Mauritus national flag problem - 4 colours,

Answer 1

This is just like the Dutch national flag problem, but we have four colors. Essentially the same strategy applies. Assume we have (where ^ represents the point being scanned).

  RRRRBBB???????????YYYYGGGG
         ^

and we scan a

1. red, then we swap the first blue with the current node
2. BLUE we do nothing
3. yellow we swap with the last ?
4. Green we swap the last yellow with the last ? Then the current node with the swapped ?

So we need to keep track or one more pointer than usual.

We need to keep track of the first blue, the first ?, the last ?, the last Y

In general, the same strategy works for any number of colors, but an increasing numbers of swaps are needed.